[linearView] Zoom out when dragging an item from the dash

When dragging windows in linear view, the workspace zooms out to
allow moving the window to other workspaces. Enable the same
behaviour for items dragged from the dash.

https://bugzilla.gnome.org/show_bug.cgi?id=618055

js/ui/genericDisplay.js

@@ -47,6 +47,14 @@ GenericDisplayItem.prototype = {
                                      }));
 
         let draggable = DND.makeDraggable(this.actor);
+        draggable.connect('drag-begin',
+                          Lang.bind(this, function() {
+                              Main.overview.beginItemDrag(this);
+                          }));
+        draggable.connect('drag-end',
+                          Lang.bind(this, function() {
+                              Main.overview.endItemDrag(this);
+                          }));
 
         this._iconBin = new St.Bin();
         this.actor.add(this._iconBin);