Blender Git Loki

NLA Channel Drawing Refactor: Remove rest of old drawing code for NLA Track channels

NLA Track drawing has now been ported over to use the UI widgets
like the rest of the anim channels do in the DopeSheet and Graph
Editors. The main benefit of this for users is that these buttons
will now show tooltips when you hover over them. Hopefully this
will help make the "solo" buttons more discoverable.

I've decided to postpone porting the "Action Line" channels to the
widget system for now, since there are quite a few more issues there
which need quite a bit more time to work through.