Also fixes some resize issues with stupid workarounds to our resize
logic. This really needs to be cleaned up at some point!

Messages on /sync don't have a room_id

(This is such a hack and will probably break, but it works for now for
most cases...)

This is necessary to support having a picker within QML.
Eventually, this should replace the existing widget-based one.

We don't get notified for every message. Sometimes we only get a read
receipt for the newest message, which means old read receipts accumulate
in the database. This least to some considerable CPU overhead, when
checking if the timeline should be notified for new read receipts.
Instead just always notify, since that has far less overhead in the
worst case and doesn't need complicated cache cleanup.

The old pending_receipts db is not removed for now. It should still have
minimal storage overhead and we don't have a good mechanism for cache
format upgrades atm.