RightPanel should remember thread panel width separately

### Your use case

#### What would you like to do?

It makes no sense that RightPanel remembers what it's displaying per room (ThreadPanel, MemberList, etc)... but doesn't remember how wide it should be. Therefore, as well as remembering contents, it should remember width too.

#### Why would you like to do it?

When viewing a room with threads, i find you rapidly expand the RightPanel as wide as possible so the ThreadPanel contents is legible. If you then switch to a room without threads, you're more likely to find the MemberList or similar open (or the RightPanel hidden entirely) - so it's very disturbing and unergonomic if the RightPanel is now sized to be 50% of the width of the screen.

### Have you considered any alternatives?

The popping of the RightPanel width as you change rooms could be a bit annoying, but given the RightPanel is 'owned' by the room in the information hierarchy, i think it's acceptable.

### Additional context

I've now resized by RightPanel about 200 times today, and i wish i hadn't had to.