Merge pull request #358 from jp9000/scene-collection-menu

Implement scene collections in menu only
Fix startup issues with scene collections

If a scene collection wasn't found at startup, it would create a
'(null)' scene due to a null pointer.

Also, moved scene collections to the scene collections folder rather
than the base folder to prevent conflicts with the older scenes.xconfig
file.
Copy scene collection to scenes.xconfig

This allows better backward compatibility with older versions.  This
will keep the current scene collection set to default if the user ever
decides to downgrade OBS versions for whatever reason.
Keep Global Sources in scenes.xconfig

Instead of each Scene Collection having its own Global Sources.
Global Sources will be stored in scenes.xconfig.

One set of Global Sources usable in all Scene Collections. Backwards
Compatible