BUG: Context menu too long (Fixes #811)

Added javascript to add and remove classes on context sub menus, to allow for multi column layout.

Linked to: https://github.com/silverstripe/silverstripe-framework/pull/2303

Tested on IE8, IE9, Chrome, and Firefox.

After:

![after](https://f.cloud.github.com/assets/984753/921736/175e4f3e-ff05-11e2-8c15-3896e1c6419d.png)