Co-authored-by: Saúl Ibarra Corretgé <saghul@jitsi.org>
Co-authored-by: zycwind <391321232@qq.com>

Contributing all buttons in one place goes against the designs that we
set out at the beginning of the project's rewrite and that multiple of
us have been following since then.

Create standalone components for each feature and move all state to them.
Toolbars are now dummy containers.

Activity.enterPictureInPictureMode method must be invoked synchronously
on userLeaveHint callback in order to be sure that the current Activity
is still visible (does not transit to PAUSED state). Previously if the
asynchronous processing would be delayed enough for the Activity to go
into the PAUSED state it will be too late to go into the PiP mode.

This only works automatically on Android >= 8. On other platforms / versions, it
relies on the SDK user on implementing a "reduced UI" mode and reacting to the
"request PIP" delegate method.

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.