Widget Customizer: Use ?wp_customize=on.

Let _wp_customize_include() handle customizer initialisation.
Sets also the priority for the Add Widget and Reorder buttons and removes debug cruft.

props westonruter.
fixes #27291.
Built from https://develop.svn.wordpress.org/trunk@27524


git-svn-id: http://core.svn.wordpress.org/trunk@27367 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-widgets.php

@@ -321,7 +321,7 @@ class WP_Customize_Widgets {
 						array(
 							'section' => $section_id,
 							'sidebar_id' => $sidebar_id,
-							//'priority' => 99, // so it appears at the end
+							'priority' => count( $sidebar_widget_ids ), // place Add Widget & Reorder buttons at end
 						)
 					);
 					$new_setting_ids[] = $setting_id;