Customize: Allow controls to be registered without any associated settings.
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.