workspaceThumbnail: Allow users to create workspaces at any position

Allow a user to create a new workspace by dragging a window or a launcher in
the middle of two existing ones.

https://bugzilla.gnome.org/show_bug.cgi?id=646409

js/ui/dash.js

@@ -231,7 +231,7 @@ DragPlaceholderItem.prototype = {
 
     _init: function() {
         DashItemContainer.prototype._init.call(this);
-        this.setChild(new St.Bin({ style_class: 'dash-placeholder' }));
+        this.setChild(new St.Bin({ style_class: 'placeholder' }));
     }
 };