Keep the almost sorted list around and sort by the raw timestamp value
instead of doing the expensive toMSecSinceEpoch conversion.

When user is mentioned (via matrix 'highlight_count'), inactive
rooms will use a different color for the notification circle than
when only general unread messages exist.

fixes #313

The old algorithm during filtering will check every room
if it's part of the group. O(N*G)

The room ids for a group are now stored in a map for faster lookup
so the search can be completed in a single pass. O(N)

fixes #2

fixes #107

fixes #128

Automatically dismiss unread notifications when the window regains
focus.

fixes #111
fixes #68

fixes #134

* Initial "join room" feature.
* React correctly to remotely joined rooms.
* Leaving rooms implemented both locally using the room menu
   in nheko, and reacting properly when leaving a room remotely 
   from another client.