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.

release/scripts/startup/bl_ui/space_outliner.py

@@ -148,6 +148,29 @@ class OUTLINER_MT_collection_view_layer(Menu):
             layout.operator("outliner.collection_holdout_clear")
 
 
+class OUTLINER_MT_collection_visibility(Menu):
+    bl_label = "Visibility"
+
+    def draw(self, context):
+        layout = self.layout
+
+        layout.operator("outliner.collection_isolate", text="Isolate")
+        layout.operator("outliner.collection_show", text="Show")
+        layout.operator("outliner.collection_hide", text="Hide")
+
+        layout.separator()
+        layout.operator("outliner.collection_show_inside", text="Show All Inside")
+        layout.operator("outliner.collection_hide_inside", text="Hide All Inside")
+
+        layout.separator()
+        layout.operator("outliner.collection_enable", text="Enable in Viewports")
+        layout.operator("outliner.collection_disable", text="Disable in Viewports")
+
+        layout.separator()
+        layout.operator("outliner.collection_enable_render", text="Enable in Render")
+        layout.operator("outliner.collection_disable_render", text="Disable in Render")
+
+
 class OUTLINER_MT_collection(Menu):
     bl_label = "Collection"
 
@@ -177,6 +200,9 @@ class OUTLINER_MT_collection(Menu):
             layout.separator()
             layout.menu("OUTLINER_MT_collection_view_layer")
 
+        layout.separator()
+        layout.menu("OUTLINER_MT_collection_visibility")
+
         layout.separator()
         layout.operator_menu_enum("outliner.id_operation", "type", text="ID Data")
 
@@ -301,6 +327,7 @@ classes = (
     OUTLINER_MT_edit_datablocks,
     OUTLINER_MT_collection,
     OUTLINER_MT_collection_new,
+    OUTLINER_MT_collection_visibility,
     OUTLINER_MT_collection_view_layer,
     OUTLINER_MT_object,
     OUTLINER_MT_context,