* Improves parity between partials and controls. A partial or control can be settingless if instantiated with `settings` param as empty array (otherwise, if null, then the partial/control ID is used).
* Eliminate need to create dummy settings that serve no purpose except to place a control in the UI.
* Removes dummy settings for `create_new_menu` and `new_menu_name`.
* Introduces `WP_Customize_Control::$capability` and `WP_Customize_Partial::$capability`, and if set checks them in the respective `check_capabilities()` methods.
* Prevents PHP fatal error from happening when non-existing settings are provided to control: "Call to a member function `check_capabilities()` on a non-object".
* Fixes issue where nav menu items and widgets were no longer working with selective refresh because cap check was failing.
See #27355.
Fixes#35926.
Built from https://develop.svn.wordpress.org/trunk@36689
git-svn-id: http://core.svn.wordpress.org/trunk@36656 1a063a9b-81f0-0310-95a4-ce76da25c4cd
