An unfortunately large set of timeline-related changes:

An unfortunately large set of timeline-related changes:

- Rename sequence_video to sequence, and sequence subtitle content
like we do video content (i.e. adding multiple subtitle contents
will result in them sequenced in time rather than overlaid).
- Stop doing selection-changed related stuff in ContentPanel
if no selection change has actually happened.
- Attempt to tidy up event handling in the timeline a bit.