Menu Customizer: trigger the customizer `processing` state, which should prevent saving while pending menu items are created. In this case, increment the processing value each time an item is added, and decrement it each time an added item has finished being added. Saving is suspended by the customizer when the processing value is 0.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@939385 b8457f37-d9ea-0310-8a92-e5e31aec5664