Customizer: Introduce a "panel" API to organize multiple sections into a one section.

Create a panel via `$GLOBALS['wp_customize']->add_panel( $panel_id, $args )` and use `$panel_id` for the `panel` argument in `$GLOBALS['wp_customize']->add_section( $section_id, $args )`. That's it.
As an example all widget area sections are now summarized into one panel. Feedback appreciated.

props celloexpressions.
see #27406.
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git-svn-id: http://core.svn.wordpress.org/trunk@28662 1a063a9b-81f0-0310-95a4-ce76da25c4cd