When auto-expanding a pane and showing it as an overlay because of
narrow window, make the chat keep the width it had before.
This makes the transition less jarring, reduces the amount of text seen
through a transparent pane, and fix making one pane overlay expanding
the other.
If the room pane is below the default width (by user resizing or because
of narrow window), go to the room settings or a member profile will
automatically expand it as needed.
The pane goes back to its previous size once the settings/profile is
left.
If the window is narrow enough that expanding the pane normally would
reduce the timeline's width past its minimum, the pane is shown as an
overlay instead.
Use native ListModel which require a lot of changes, but should be
much faster than the old way which exponentially slowed down to a crawl.
Also fix some popup bugs (leave/forget).
Not working yet: side pane keyboard controls, proper highlight,
room & member filtering, local echo replacement
