Reorder styles.
Cursor pointer on progress bar.
UI reorganization.
Add a scheduler idea comment.
Overhaul transport event generation and looping.

* Separate "real time" display time and "gen time" internal time. This
  allows a clean implementation of lookahead generation involving seeks.

* Looping is no longer a special case; now handled by SeekEvent which
  uses event.finalize under the hood.

* Customizable lookahead duraton.