Add theme browsing and theme switching to the Customizer

* Brings into core the Customizer Theme Switcher feature plugin
* You can now browse, preview, and activate themes right from the Customizer

fixes #31303.
props celloexpressions, afercia, westonruter, folletto, designsimply

git-svn-id: https://develop.svn.wordpress.org/trunk@31533 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-section.php

@@ -311,6 +311,57 @@ class WP_Customize_Section {
 	}
 }
 
+/**
+ * Customize Themes Section Class.
+ *
+ * A UI container for theme controls, which behaves like a backwards Panel.
+ *
+ * @package WordPress
+ * @subpackage Customize
+ * @since 4.2.0
+ */
+class WP_Customize_Themes_Section extends WP_Customize_Section {
+
+	public $type = 'themes';
+
+	/**
+	 * Render the themes section, which behaves like a panel.
+	 *
+	 * @since 4.2.0
+	 */
+	protected function render() {
+		$classes = 'accordion-section control-section control-section-' . $this->type;
+		?>
+		<li id="accordion-section-<?php echo esc_attr( $this->id ); ?>" class="<?php echo esc_attr( $classes ); ?>">
+			<h3 class="accordion-section-title" tabindex="0">
+				<?php echo esc_html( $this->title ); ?>
+				<span class="screen-reader-text"><?php _e( 'Press return or enter to expand' ); ?></span>
+			</h3>
+			<span class="control-panel-back themes-panel-back" tabindex="-1"><span class="screen-reader-text"><?php _e( 'Back' ); ?></span></span>
+			<div class="customize-themes-panel control-panel-content themes-php">
+				<h2><?php esc_html_e( 'Themes' ); ?>
+					<span class="title-count theme-count"><?php echo count( $this->controls ) - 1; ?></span>
+				<?php if ( ! is_multisite() && current_user_can( 'install_themes' ) ) : ?>
+					<a href="<?php echo admin_url( 'theme-install.php' ); ?>" target="_top" class="add-new-h2"><?php echo esc_html_x( 'Add New', 'Add new theme' ); ?></a>
+				<?php endif; ?>
+				</h2>
+				<div class="theme-overlay" tabindex="0" role="dialog" aria-label="<?php esc_attr_e( 'Theme details' ); ?>"></div>
+				<div id="customize-container"></div>
+				<?php if ( 6 < count( $this->controls ) ) : ?>
+					<p><label for="themes-filter">
+						<span class="screen-reader-text"><?php _e( 'Search installed themes...' ); ?></span>
+						<input type="search" id="themes-filter" placeholder="<?php esc_attr_e( 'Search installed themes...' ); ?>" />
+					</label></p>
+				<?php endif; ?>
+				<div class="theme-browser rendered">
+					<ul class="themes accordion-section-content">
+					</ul>
+				</div>
+			</div>
+		</li>
+<?php }
+}
+
 /**
  * Customizer section representing widget area (sidebar).
  *