Customize: Fix regressions and harden implementation of selective refresh for nav menus.

* Request full refresh if there are nav menu instances that lack partials for a changed setting.
* Restore `WP_Customize_Nav_Menus::$preview_nav_menu_instance_args` and `WP_Customize_Nav_Menus::export_preview_data()` from 4.3, and keeping a tally of all `wp_nav_menu()` calls regardless of whether they can use selective refresh.
* Ensure that all instances of `wp_nav_menu()` are tallied, regardless of whether they are made during the initial preview call or during subsequent partial renderings. Export `nav_menu_instance_args` with each partial rendering response just as they are returned when rendering the preview as a whole.
* Fix issues with Custom Menu widget where nav menu items would fail to render when switching between menus when a menu lacked items to begin with.
* Make sure the fallback behavior is invoked when the partial is no longer associated with a menu.
* Do fallback behavior to refresh preview when all menu items are removed from a menu.

Follows [36586].
See #27355.
Fixes #35362.

Built from https://develop.svn.wordpress.org/trunk@36889


git-svn-id: http://core.svn.wordpress.org/trunk@36856 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.5-beta2-36888';
+$wp_version = '4.5-beta2-36889';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.