* Make useEventEmitterState more efficient
By not invoking the initializing function on every render
* Make useWidgets more efficient
By not calling WidgetStore on every render
* Add new group call experience Labs flag
* Add viewingCall field to RoomViewStore state
Currently has no effect, but in the future this will signal to RoomView to show the call or call lobby.
* Add element_call.use_exclusively config flag
As documented in element-web, this will tell the app to use Element Call exclusively for calls, disabling Jitsi and legacy 1:1 calls.
* Make placeCall return a promise
So that the UI can know when placeCall completes
* Update start call buttons to new group call designs
Since RoomView doesn't do anything with viewingCall yet, these buttons won't have any effect when starting native group calls, but the logic is at least all there and ready to be hooked up.
* Allow calls to be detected if the new group call experience is enabled
* Test the RoomHeader changes
* Iterate code
