The "Mentions & keywords only" option doesn't work yet in multi-account
cases: when notifications arrive for a room that multiple accounts are
part of, the first notification to arrive is let through, and the
duplicate later ones are ignored. The first notification to arrive may
not be from the account that gets highlighted, thus it is ignored under
the "Mentions & keywords" setting.
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
