dash: Reset show-apps label on drag-end

Currently the label for the show-apps button is only updated during
drag operations, so after an item is successfully dropped on the
button, the label will still read "Remove from Favorites".
Fix this by resetting the label on drag-end.

https://bugzilla.gnome.org/show_bug.cgi?id=684627

js/ui/dash.js

@@ -446,6 +446,7 @@ const Dash = new Lang.Class({
 
     _endDrag: function() {
         this._clearDragPlaceholder();
+        this._showAppsIcon.setDragApp(null);
         DND.removeDragMonitor(this._dragMonitor);
     },