* Ensure that a Custom Menu widget selecting a newly-inserted menu gets updated to use the new menu ID upon Save & Publish.
* Dynamically update the visibility of the Custom Menu widget's "no menus" message when the number of menus changes between 0 and 1+.
* Send all dirty Customized settings in `update-widget` Ajax request and `preview()` them so that the widget update/form callbacks have access to any data dependencies in the current Customizer session (such as newly created unsaved menus).
* Update link in Custom Menu widget to point to Menus panel as opposed to Menus admin page, when in the Customizer.
* Fix an issue with extra space at top immediately after creating new menu.
* Fix doubled `update-widget` Ajax requests when changing select dropdown; prevent initial from being aborted.
* Add missing `wp_get_nav_menus()` hooks to preview Customizer updates/inserts for `nav_menu` settings; includes tests.
* Update `wp_get_nav_menu_object()` to allow a menu object to be passed in (and thus passed through).
Props westonruter, adamsilverstein.
Fixes#32814.
git-svn-id: https://develop.svn.wordpress.org/trunk@33488 602fd350-edb4-49c9-b593-d223f7449a82
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