Added "Import" button in Global Sources Dialog.

The "Import" button opens a "Open" dialog where a user can open a scene collection file then it lists all Global Sources from the selected scene collection in a dialog.

You can Import multiple global sources by holding shift or Ctrl and clicking on the Global sources you 
want to import.

With this change scene collections have their own Global Sources but users can import Global Sources from other scene collections.

If you make any changes to a Global sources properties and want the change in another scene collection you will need to remove the global source in the other scene collection and 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.