Blender Git Loki

Hack: Use same handling logic for finding "active" GPencil block in timeline as for 3D View

This is not a terribly nice hack, but it gives the behaviour that users expect
more often than not. Namely, this enables:
* GPencil keyframes now show up in timeline
* The next/prev keyframe buttons on the Timeline header work correctly
(i.e. they can "see" the GPencil keyframes)