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