Add User Mentions Dialog

Add a RoomListItem-like button that opens a dialog
containing all of the messages that would result in a
highlight from the server (for example, the user is mentioned,
or @room is mentioned).

This is VERY rudimentary and will be completely reworked in the future
to take advantage of the existing TimelineView class, instead of
using a dialog like it does now. The button to show the mentions
also needs work.