Theme Customizer: Allow the customize iframe to be accessed directly (with...

Theme Customizer: Allow the customize iframe to be accessed directly (with full feature support). see #19910.

* Move the 'Return to Manage Themes' and 'Collapse Sidebar' actions from themes.php to customize-controls.php.
* Create a postMessage connection between themes.php and customize-controls.php.
* Allow the theme customizer to be accessed directly (independent of themes.php and the customize loader).
* Add wp_customize_href() and wp_customize_url().
* Remove wp_customize_loader(). To include the loader, use wp_enqueue_script( 'customize-loader' ).
* The theme customizer now requires postMessage browser support.
* Add .hide-if-customize and .hide-if-no-customize CSS classes.
* Clean up customize-preview.js.

git-svn-id: https://core.svn.wordpress.org/trunk@20476 1a063a9b-81f0-0310-95a4-ce76da25c4cd