Implement cross overview drag & drop

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

js/ui/workspacesView.js

@@ -644,6 +644,9 @@ WorkspacesView.prototype = {
     },
 
     _onDragMotion: function(dragEvent) {
+        if (Main.overview.animationInProgress)
+             return DND.DragMotionResult.CONTINUE;
+
         let primary = global.get_primary_monitor();
 
         let activeWorkspaceIndex = global.screen.get_active_workspace_index();