Customizer: Introduce customize_loaded_components filter to allow core components to be disabled.

Also move style rule from `customize-nav-menus.css` to `customize-controls.css` so that widgets button is properly styled when `nav_menus` component is excluded from loading. See [35304]. See #33327.

Props westonruter, DrewAPicture.
Fixes #33552.

Built from https://develop.svn.wordpress.org/trunk@35307


git-svn-id: http://core.svn.wordpress.org/trunk@35273 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-controls-rtl.css

@@ -3,6 +3,18 @@ body {
 	-webkit-text-size-adjust: 100%;
 }
 
+button.not-a-button {
+	background: transparent;
+	border: none;
+	-webkit-box-shadow: none;
+	box-shadow: none;
+	-webkit-border-radius: 0;
+	border-radius: 0;
+	outline: 0;
+	padding: 0;
+	margin: 0;
+}
+
 #customize-controls a {
 	text-decoration: none;
 }