Begin yet another model refactor

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

src/backend/qml_bridge.py

@@ -110,8 +110,6 @@ class QMLBridge:
 
         rc = lambda c: asyncio.run_coroutine_threadsafe(c, self._loop)  # noqa
 
-        from .models.items import Account, Room, Member, Event, Device  # noqa
-
         p = print  # pdb's `p` doesn't print a class's __str__  # noqa
         try:
             from pprintpp import pprint as pp  # noqa