Initial On Screen Controller implementation

Implements a simple on screen controller in lua which provides basic playback controll.
Removed some old debug stuff
More cleanups
prevent bar filling from rendering negative positions
Render chapter buttons as disabled when there are no chapters
command: add ratio-pos property

More precise than percent-pos. It would be nicer to have the percent-pos
property return a float instead, but that would make things look ugly if
the property is used for OSD.
pretend that ass draw stuff is a lua package