Call a hook when updating a queue

Because of the changes introduced in #234004: Drag and drop node ordering (by myself!) it is no longer possible for other modules to react to updates to subqueues such as reordering nodes. I'm working on a simple patch to add a hook_nodequeue_update, which will be called when submitting the node reordering form.

This change is necessary because the drag-and-drop interface doesn't use the nodequeue_queue_up/down functions, which invoke the nodequeue_swap hook.