* feat(multi-stream-support) Add screenshare as a second video track to the call.
This feature is behind a sendMultipleVideoStreams config.js flag. sourceNameSignaling flag also needs to enabled. Sending multiple tracks is currently supported only on endpoints running in unified plan mode. However, clients with source-name signaling enabled and running in plan-b can still receive multiple streams .

* squash: check if there is an existing track before adding camera/desktop

* squash: enable multi-stream only on unified plan endpoints.

* feat: Use source name based receiver constraints

* add sourceNameSignaling feature flag to config

* move source name specific variables into if block

* ensure sourceName is defined in constraints

* use source name for selectedSources

* use selector to find video track by participant id

* add selector to encapsulate logic to get track source name

* refactor getTrackSourceNameByMediaTypeAndParticipant selector

* rename variable

* move flags config into Feature Flags section

* do not set constraints for local large video

* rename prioritizedSources to onStageSources

* fix flow error

* refactor visibleRemoteTrackSourceNames to a constant

* use selector to get feature flags

* rename selector function

* fix flow error

* add selector for sourceNameSignaling feature flag

- Remove button list from interface_config.js since it has been deprecated for a
  while
- Alphabetically sort buttons in config.js and constants.js to make it easier to
  add / remove items
- Add missing invite and toggle-camera buttons to default constants
- Remove no longer existing "fodeviceselection" button

Fixes: https://github.com/jitsi/jitsi-meet/issues/9605

* feat(toolbox) allow any toolbox button to be displayed as main

fixes the previous behaviour where only a certain set of buttons were whitelisted for being displayed in the main toolbar

* code review

* code review - fix avatar icon position

* audio only screen share implementation

* clean up

* handle stop screen share from chrome window

* update icon

Knowledge is power, man!

The config.js cache predates the feature base/known-domains.
Technically, it's also able to recall more domains that the feature
recent-list can (because the latter limits its entries).