Local View

Bring back per-viewport localview. This is based on Blender 2.79.
We have a limit of 16 different local view viewports.

We are using both the numpad /, as well as the regular /.

Missing features:
* Hack to make sure lights are always visible.
* Make rendered mode with external engines to support this as well
  (probably just need to support this in the RNA iterators).
* Support over 16 viewports by taking existing viewports out of local view.

The code can use a cleanup pass in the future to unify the test to see
if an object is visible (or we can use TESTBASE in more places).

source/blender/editors/include/ED_uvedit.h

@@ -41,6 +41,7 @@ struct ImageUser;
 struct Main;
 struct Object;
 struct Scene;
+struct View3D;
 struct ViewLayer;
 struct SpaceImage;
 struct bNode;