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

closes #94

Keep the almost sorted list around and sort by the raw timestamp value
instead of doing the expensive toMSecSinceEpoch conversion.

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.

This turns `paginationInProgress` field of `TimelineModel` into a `Q_PROPERTY`, so the Ui can bind to it.
For the moment, I'm showing the same spinner as we do during initial sync. It's not ideal, on the count of being giant and in the middle but it's better than nothing. We can make it more subtle later.

Fixes #180

Sometimes thumbnails still have the wrong dimensions, as they are scaled
to fit inside a rectange of the reported size in the image. Not sure,
who is wrong there, the media repo or we.