fixes #214

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

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.

fixes #172

This may increase the db size by a factor of 1000 in the worst case and
it will need some fixes, when we decide to not show some events in the
timeline, but it should work for now.