NLA SoC: Little optimisation + Drawing bugfix

NLA SoC: Little optimisation + Drawing bugfix

* Text labels on NLA-Strips should now draw properly for most short-strips now. Previously, the padding on the text was a bit too extreme, so for very short strips (less than 4 frames or so), the text was often pushed down into the bottom-right corner of view.

* Optimised the keyframe-highlighting code for buttons a bit. Replaced the custom linear-search with the binary-search used when inserting keyframes (and for the 3d-view keyframe-indicator). There should be some theoretical improvements due to this at least...