Lock position of the room that's focused in GUI

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).

docs/CHANGELOG.md

@@ -86,6 +86,11 @@ and this project adheres to
 
 - Improve room page loading speed 
 
+- In the left pane, lock the position of the room corresponding to the 
+  currently visible chat page if any.  
+  This fixes annoyances like clicking on a room with unread messages only to 
+  see it immediatly fly down the list, potentially outside of scrolling view.
+
 - When replying to a message, pressing the reply keybind again while focusing
   on that message will now cancel the reply