Theme Customizer: Improve the customize loader and themes list table markup. see #19910.

* Use event delegation for triggering the customize loader (so we play nicely with infinite scroll).
* Use data attributes on .load-customize links instead of parsing the href.
* Properly translate the 'Customize' string instead of injecting the replacement with JS.

git-svn-id: https://core.svn.wordpress.org/trunk@20352 1a063a9b-81f0-0310-95a4-ce76da25c4cd