New frame bar.

Adds a FrameIndex that tracks frames better. This removes the nasty
copy/paste logic for duration calculation everywhere that was causing
precision issues. The frame bar is now shown above the zone that contains
it, instead of globally. This will allow multiple zones with frame timings in the future.

Some refactoring of commands/keyboard bindings to make them easier to find.
Future commits will work on the nav bar - for now, the frames have been removed.

Fixes #184.