Menu Customizer: work on the add-menu-item functionality and related fixes:
- Clean up MenuItemModel declaration to remove unneeded and duplicate properties.
- When a new item is added and returned from the server, register a new setting and control for it before rendering it and making it editable.
- Lay the groundwork for a css-based "bouncing" animation to give emphasis to newly added items once they become editable (needs work on applying the needed class at the appropriate time). Replaces a slow fadeIn from nav-menu.js.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@935946 b8457f37-d9ea-0310-8a92-e5e31aec5664