Blender Git Loki

Layer collection enable flag

Right now this is exposed in the outliner, though all this
(visible/selectable/enable) should be moved to a new panel soon.

This removes objects from the depsgraph when the collection is disabled.

It allows you to "hide" lamps but still having them lighting the scene.
Same for light probes and other support objects.

Pending tasks:

* Have depsgraph to include invisible objects in the DEG_OBJECTS_ITER, and
then have Eevee and other engines to make a distinction between an
invisible and a visible object.

(for example, we probably want invisible objects to not show in the
viewport, but cast shadows and show up in light probes).

* Change how we evaluate collection settings so that an invisible
collection can force an object to be invisible.

Reviewers: campbellbarton

Subscribers: sergey

Differential Revision: https://developer.blender.org/D2848