timeline: Remove dead code
mediafilespreviewer: Remove sync-message handler and add proper xid handling

Sync message handlers tend to slow down Pitivi, and the xid is no longer saved
in the preview widget as this was racy. Eventually, we should refactor this
so that all the handling of GstVideoOverlay is done in only one place.
Kill ChangeLog with fire

This is not 1999. We have working version control now.
Releasing Pitivi 0.91 "Charming Defects"