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).
