Merge pull request #339 from Glought/master

Added "Scene Collection" menu to switch between different scene collections.
Fix crash when deleting a global source that didn't load
Don't allow runtime changing of scene collections as this leaves dangling XElement pointers in all the current SceneItems which causes crashes.
Refresh the Sources list when changing scene collections.
Disabled changing scene collections in settings.

Disabled changing, adding , renaming, and removing scene collections in
Settings General while streaming.