Avoid re-rendering RoomList on room switch

[dbkr]

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. This basically re-introduces concepts from the ConstantTimeDispatcher branch.

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.

Requires element-hq/element-web#5015

[ara4n]

lgtm, although the fact that ActiveRoomObserver is not a generic Observer makes me feel very very sadpanda. it's not a blocker for merging it, but i can see this pattern being used all over the shop to save CPU (e.g. i'm sure MemberList suffers the same woes, especially if presence were turned on), and having the impl concretized into the ActiveRoom specific scenario feels Wrong.

Also, i'm assuming you've tested that the CallPreview persists correctly if you're on a 1:1 when switching between rooms (and when switching between preview and non-preview mode), as istr this was slightly subtle when I wrote it. It would suck if the preview gets ripped down and remounted every time you changed room or something.

[dbkr]

Yeah - intention here is very much that if we introduce more of these, we can move the common functions into their own class.

And yep, CallPreview does the right thing in my testing, and stays nicely in the corner when switching between rooms other than the one the call is in (and also correctly goes away if the call gets hung up from the other end).