When in drag mode, the linear view raises a transparent actor
covering the entire stage. That way the view can handle events
like dragging items to the screen edges, but in order to not
disable any drop target not handled by itself, it duplicates
huge amounts of code from dnd.js.
Cleanup that mess by using the new drag monitoring support.
https://bugzilla.gnome.org/show_bug.cgi?id=620504