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.

git-svn-id: https://develop.svn.wordpress.org/trunk@27524 602fd350-edb4-49c9-b593-d223f7449a82