workspaces: Change handling of window-drag signals

Delegate the emission of the window-drag-begin/window-drag-end
signals to overview functions, as done already for other items.
This will enable objects to react to those signals without having
access to the workspace objects / the workspaces view.

https://bugzilla.gnome.org/show_bug.cgi?id=634948

js/ui/overview.js

@@ -247,6 +247,14 @@ Overview.prototype = {
         this.emit('item-drag-end');
     },
 
+    beginWindowDrag: function(source) {
+        this.emit('window-drag-begin');
+    },
+
+    endWindowDrag: function(source) {
+        this.emit('window-drag-end');
+    },
+
     // Returns the scale the Overview has when we just start zooming out
     // to overview mode. That is, when just the active workspace is showing.
     getZoomedInScale : function() {