When the currently shown page is the chat of a certain room,
prevent that room from moving around in the left pane due to new
messages/activity or unread/highlight counters change.
When the user switches to another page/room, the previously held lock is
released and that room completes all the moves it would have done if it
wasn't locked.
This makes navigating a room list with lots of activity easier, and
prevent annoyances like clicking on a room with unread messages and
having it immediatly fly down the list (possibly out of scroll view).
This reverts commit 39f159f0a3.
Causes problems when message delegates are reordered, with the
transition animations randomly stopping in the middle and leaving
delegates at odd positions.
