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.