Blender Git Loki

Support multi-window setups nicely

Opening new windows now never duplicates the workspace, you can now
show the same workspace in multiple windows. The layout is per window
though, so you can have the same workspace active in multiple windows,
but use a different active layout.

Opening windows behaves as follows now:
* Adapt behavior of rB7bc76f8a3c1416b
* Duplicating an area into a new window creates a new screen-layout, we agreed this is probably what users would want.
* Creating a new temporary window (e.g. UserPrefs) adds a new hidden screen-layout
* Enabling multi-view time-sequential creates a new window but doesn't duplicate screen-layout anymore