Blender Git Loki

Collections: Initial support for animating/driving collection properties (T55233)

(Just committing this now to a temp branch so that I can continue working on this
from another machine later. This is a re-based+squashed, re-pushed version of what
I just pushed earlier, but now based on current 2.8 code, not from several days ago)

Rationale:
The Spring team needs a way to hide objects from the viewport, so that parts of
the rig can be enabled/disabled per shot. An example of this is how the cornea
meshes are typically hidden from the viewport so that the animators can see
the irises, and hence, where the character is looking.

(Another reason we may want this in future is to make it so that a bunch of
objects/rigs can be keyframed together in the same action, making it easier
to manage their actions)

Status:
* Currently all necessary data and animation editor support changes should be
in place and working. Hopefully I haven't missed any - the checklist may need
updating for 2.8

* Depsgraph support however is still incomplete. We still need to figure out what
needs to happen with the animated values to make objects actually appear/disappear
when triggered via the animation system, just like they do now from the UIwip