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.

git-svn-id: https://develop.svn.wordpress.org/trunk@28861 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/customize.php

@@ -142,8 +142,12 @@ do_action( 'customize_controls_print_scripts' );
 
 			<div id="customize-theme-controls"><ul>
 				<?php
-				foreach ( $wp_customize->sections() as $section )
+				foreach ( $wp_customize->panels() as $panel ) {
+					$panel->maybe_render();
+				}
+				foreach ( $wp_customize->sections() as $section ) {
 					$section->maybe_render();
+				}
 				?>
 			</ul></div>
 		</div>