Fix writing of empty strings
Added "Import" button in Global Sources Dialog.

The Main Scene Collection is "sceneCollection\scenes.xconfig" since it is the original user's scenes file.

The "Import" button lists all Global Sources from the main scene collection.

With this change scene collections have their own Global Sources but users can import Global Sources from the main scene collection.

You can't import Global Sources into the main scene collection. You can only import Global Sources from the main scene collection into other scene collections.

If you want to make a Global Source that you will use a lot like for example Follower alerts, Subscriber alerts, etc add it to the main scene collection so you can import it to other scene collections.

If you make any changes to the Global sources properties in the main scene collection it will not effect Global sources already imported in other scene collections you will need to remove the Global Source then import it again.

You can't import Global Sources if there is already a Global Source with the same name.

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