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".

Hopefully this makes it simple and fast for users to import their commonly used Global sources like for example followers alerts.

I'm using Microsoft visual Studio express 2013.  What is the proper spaced indentation?