new types make it possible for layouts to be stacked on top of each
other with correct focus behavior.
- Tweaks to the volume and time slider to make them more accurate.
- Fixed a bug in the main event loop to re-check focus to make sure it
hasn't changed under the hood since the last pass through.
- Updated transport volume control style
- Cleaned up and rearranged some input handling so we can better support
nested layouts in the future
- Tweaked Window to use panels so draw ordering can be vaguely
controlled, at least as well as vanilla ncurses
- Started working on overlay functionality, so layouts can do things
like display dialogs.