While we probably won't be getting a general purpose "Onion Skinning"
overlay anytime soon for meshes, etc. (at least not before the
depsgraph stabilises, and we also get geometry caching working),
for the Grease Pencil integration at least, it makes sense to move
GP objects to using a more general/future-proof solution, instead
of continuing to use a special/dedicated button in the header.
Currently the UI part of this is commented out. Also, the GP branch
doesn't need to move to this pre-merge. But, since 2.8 changes move
fast, it's better to reserve the space now to have it next to motionpaths,
than introduce it later.
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