Blender Git Loki

Handle multi-hidden-layers when converting groups to collections.

That is kind of mandatory with complex rigged-character groups, with
hundreds of helper objects, and a few useful ones being hidden on
specific layers (like e.g. the main rig...).

It is especially critical point with static override, which won't allow
to move objects between collections and such (that would be a nightmare
to implement and handle).

Note that this is rather basic implementation, we could go further and
move all objects in all layers they are 'active', but that would
probably be overkill.

Reviewers: brecht

Subscribers: brecht

Differential Revision: https://developer.blender.org/D3649