Blender Git Loki

Outliner visibility unification: Implement 3 levels of viewport visibility

Now collection and objects can be either:
* Disabled for all the view layers.
* Hidden for a view layer but not necessarily for all others.
* Visible for a view layer but not necessarily for all others.

Regarding icons: Whatever we decide to use for the "Hidden for all view
layers" needs to be a toggle-like icon. Because when viewing "Scenes"
instead of "View Layer" in the outliner we should be able to edit the
collection "Hidden for all the view layers" as an on/off option.

The operators are accessible via a Visibility context menu or shortcuts:
* Ctrl + Click: Isolate collection (use shift to extend).
* Alt + Click: Disable collection.
* Shift + Click: Hide/Show collection and its children (objects and collections)

Things yet to be tackled:
* Object outliner context menu can also get a Visibility sub-menu.
* Get better icons for viewport enable/disable.

Note:
* When using emulate 3 button mouse alt+click is used for 2d panning.
In this case users have to use the operator from the menu.

See T57857 for discussion.

Patch: https://developer.blender.org/D4011
Reviewers: brecht and sergey

Thanks to the reviewers and William Reynish and Julien Kasper in
particular for the feedback.