Widget Customizer: Improve plugin compatibility.

Some plugins are using custom scripts and styles for there widgets. These are available on the Widgets screens, but not in the Customizer yet.
Scripts and styles can be enqueued via: 
* `admin_enqueue_scripts`
* `admin_print_scripts` and `admin_print_scripts-widgets.php`
* `admin_print_styles` and `admin_print_styles-widgets.php`
* `admin_print_footer_scripts` and `admin_footer-widgets.php`
All this hooks are now called in the Customizer too.

Previously we have add the `#widgets-right` ID to a container div via jQuery. Remember: `#widgets-right` exists on the Widgets screen and is used by many plugins to do event delegation from that element.
But since our script files are loaded in the footer, the JavaScript way is a bit late for some plugins.
We have decided to add a `div#widgets-right` container element to customizer. "Less hacky hack."

props westonruter, ocean90. Thanks dpe415 for testing.
fixes #27619.

git-svn-id: https://develop.svn.wordpress.org/trunk@27907 602fd350-edb4-49c9-b593-d223f7449a82