Added "Import" button in Global Sources Dialog.

The "Import" button lists all Global Sources from
"sceneCollection\scenes.xconfig".

With this change scene collections have their own Global Sources but users can import Global Sources from "sceneCollection\scenes.xconfig".
The code formatting here seems a bit weird.  Actually, it seems like you're using 3-space indentation in a lot of places, but here in particular is an example of some really strange looking code.