The gnome-panel allows the user to hover over a tasklist entry
while draging to activate a minimized or obscured window and drop onto it.
Implement a similar behaviour by allowing draging to the activities button or
the hotcorner (and thus opening the overview), which allows the user to
activate any window (even on different workspaces) as a drop target.
https://bugzilla.gnome.org/show_bug.cgi?id=601731
MutterWindow and MutterPlugin have been renamed to MetaWindowActor
and MetaPlugin, mutter_plugin_list_windows() to
meta_plugin_list_window_actors(). Adapt to those changes.
https://bugzilla.gnome.org/show_bug.cgi?id=632500
