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.