* 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.

* add button state / helper dialog

* new audio screen share flow

* change error message

* address a couple of merge conflicts

* fix lint

* address code review

* address code review

* restrict audio screen share on mobile browsers

* remove redundant event / only show on supported env

* remove unused imports

* audio only screen share implementation

* clean up

* handle stop screen share from chrome window

* update icon