Blender Git Loki

Graph editor: Add channel option to make it persistent on display

The idea is to allow certain animation channels to be always visible in
animation editors. So, for example, one can pin Camera animation to the
editor so it is always possible to refine/tweak camera animation when
animating something else in the scene.

There is probably some more polishing required, and some current
limitations could be solved in the future but should be a good starting
point already.

Currently only works for object without recursing into deeper datablock
(so for example, it's not possible to pin object material animation).

Studio request by Colin Levy.