Ensure that newly-added posts/pages appear among list of available nav menu items

[westonruter]

• Ensure that newly-created posts/pages appear among the nav menu items. Need to ensure customized state is included in search-available-menu-items-customizer and load-available-menu-items-customizer requests. Beware of effort to combine these requests into one, as well.
• Ensure that newly-created pages get prepended to the list of available nav menu item pages if the Ajax request has already been made.
• Non-published posts/pages should be removed from available nav menu items.
• Posts/pages with changed titles should have those title changes reflected in the available nav menu items.
• Additions/changes/removals to posts/pages should be reflected when searching for nav menu items.
• When a page title is edited, make sure the corresponding available item and the “original” link show the updated title.

Closely related to #49, #147.

[westonruter]

Code can be gleaned from https://core.trac.wordpress.org/changeset/38436