We're already using the notion of _WILL_ in redux action types and
there's currently no compelling reason to introduce _BEGIN_ as well.

While reviewing "[PREVIEW|RN]: Handle getUserMedia in progress" I
discovered JSDoc comments which could be improved. They are not
necessarily 100% related to the PR.

This commit adds extra actions/Redux state to be able to deal with
the GUM operation being in progress. There will be early local track
stub in the Redux state for any a local track for which GUM has been
called, but not completed yet.

Local track is considered valid only after TRACK_ADDED event when it
will have JitsiLocalTrack instance set.

* Javadoc introduced @code as a replacement of <code> and <tt> which is
  better aligned with other javadoc tags such as @link. Use it in the
  Java source code. If we switch to Kotlin, then we'll definitely use
  Markdown.

* There are more uses of @code in the JavaScript source code than <tt>
  so use @code for the sake of consistency. Eventually, I'd rather we
  switch to Markdown because it's easier on my eyes.

* Xcode is plain confused by @code and @link. The Internet says that
  Xcode supports the backquote character to denote the beginning and end
  of a string of characters which should be formatted for display as
  code but it doesn't work for me. <tt> is not rendered at all. So use
  the backquote which is rendered itself. Hopefully, if we switch to
  Markdown, then it'll be common between JavaScript and Objective-C
  source code.

When do we need tracks?

- Welcome page (only the video track)
- Conference (depends if starting with audio / video muted is requested)

When do we need to destroy the tracks?

- When we are not in a conference and there is no welcome page

In order to accommodate all the above use cases, a new component is introduced:
BlankWelcomePage. Its purpose is to take the place of the welcome page when it
is disabled. When this component is mounted local tracks are destroyed.

Analogously, a video track is created when the (real) welcome page is created,
and all the desired tracks are created then the Conference component is created.
What are desired tracks? These are the tracks we'd like to use for the
conference that is about to happen. By default both audio and video are desired.
It's possible, however, the user requested to start the call with no
video/audio, in which case it's muted in base/media and a track is not created.

The first time the app starts (with the welcome page) it will request permission
for video only, since there is no need for audio in the welcome page. Later,
when a conference is joined permission for audio will be requested when an audio
track is to be created. The audio track is not destroyed when the conference
ends. Yours truly thinks this is not needed since it's a stopped track which is
not using system resources.

The name better suits its purpose, since it can be called at any time.

* ref: video muted state

Get rid of 'videoMuted' flag in conference.js

* ref: audio muted state

Get rid of 'audioMuted' flag in conference.js

* fix(conference.js|API): early audio/video muted updates

* ref(conference.js): rename isVideoMuted

Rename isVideoMuted to isLocalVideoMuted to be consistent with
isLocalAudioMuted.

* doc|style(conference.js): comments and space after if

* ref: move 'setTrackMuted' to functions

* fix(tracks/middleware): no-lonely-if

* ref(features/toolbox): get rid of last argument

* ref(defaultToolbarButtons): rename var

Simplify the code by using a bitfied instead of a couple of boolean flags. This
allows us to mute the video from multiple places and only make the unmute
effective once they have all unmuted.

Alas, this cannot be applied to the web without a massive refactor, because it
uses the track muted state as the source of truth instead of the media state.

Listeners were set for when a track muted or changed its video
type, but the listeners were never removed. This would could
cause events to keep firing on the removed tracks, which would
cause redux to fire and error because the tracks were no longer
known. That the tracks still fire events after removal is
another issue...

- Add tracks to the redux store by intercepting where the
  tracks actually get used via conference.replaceTrack
- While the replace call is unique to web, the _dispose and
 _addTracks calls use existing native code implementations
- Between _dispose and addTracks is a call to update mute state.
  Without it, when changing devices or videoType while muted,
  the user will stay muted (whereas existing web behavior
  causes unmute). This is due to middelware calling
  _syncTrackMutedState to make the track mute if the user is
  currently muted.
- Move the rest of ConferenceEvents.TRACK_MUTE_CHANGED into
  middleware so the event is no longer used
- Note: This change does not guarantee the track state in the
  redux store will be 100% accurate, specifically the attribute
  videoStarted. Muted and videoType should be accurate.

When a new local video track is created an associated video capturer is created
for it. The cause for the freezes seems to be creating mutliple tracks (which
come with a video capturer each). Fix this by first disposing of the previous
video track before creating the new one.

Ref:
https://github.com/oney/react-native-webrtc/issues/209#issuecomment-281482869

Lib-jitsi-meet does not really implement isScreenSharing. Besides,
getCameraFacingMode will already make sure that the video track does not
represent a desktop stream.

Until we make a decision on access modifier hints and adopt a respective
coding style, consistency is king.

An error was discovered and fixed by GitHub user blackneck in
jitsi/jitsi-meet PR #1017. The faulty source code was a piece of
duplication though. Remove the source code duplication there to reduce
the risks of bugs.

As an intermediate step on the path to merging jitsi-meet and
jitsi-meet-react, import the whole source code of jitsi-meet-react as it
stands at
2f23d98424
i.e. the lastest master at the time of this import. No modifications are
applied to the imported source code in order to preserve a complete
snapshot of it in the repository of jitsi-meet and, thus, facilitate
comparison later on. Consequently, the source code of jitsi-meet and/or
jitsi-meet-react may not work. For example, jitsi-meet's jshint may be
unable to parse jitsi-meet-react's source code.