Remove DND from top window group to allow dragging to the windowlist

This fixes issues when dragging a file in Nemo to another Nemo window
or a tab in Firefox to another Firefox window through the windowlist.
Merge pull request #82 from AlbertJP/dnd-firefox

Remove DND from top window group to allow dragging to the windowlist
1.8.1
Merge remote-tracking branch 'upstream/master'