Introduce a class that consumes updates from the RoomViewStore and
announces to listeners if the active room ID is now or is no longer
the room ID they specified. Naming suggestions welcome: it's
currently called ActiveRoomObserver.
Avoids passing the selectedRoomId down from MatrixChat all the way
through the LeftPanel / RoomList / RoomSubList to the RoomTiles.
Also introduce a CallPreview class that listens directly for
RoomViewStore changes as the call preview in the left panel needs
to know when the room changes, so this allows this component to
update without having to update the entire left panel.
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