Previously, events for which the sender, target (state_key) or remover
was missing from the room members would have their profile fetched
from network when registering the event into models.
This could cause very slow past events loading times for rooms, since
the event registering function (which contained the profile retrieval
directives) is run sequentially event-by-event.
Missing profiles are now lazy-loaded when events come into the
user's view in the QML timeline.
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
Is now strictly about setting up asyncio/uvloop and providing the
functions for QML to interact with it and call backend coroutines.
The appdirs attribute is moved to Backend.
