Themes: Pass information about the old theme in the form of a `WP_Theme` object when the `switch_theme` action is fired.

Props MikeHansenMe.
See #22401.

Built from https://develop.svn.wordpress.org/trunk@36502


git-svn-id: http://core.svn.wordpress.org/trunk@36469 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -739,15 +739,18 @@ function switch_theme( $stylesheet ) {
 	}
 
 	update_option( 'theme_switched', $old_theme->get_stylesheet() );
+
 	/**
 	 * Fires after the theme is switched.
 	 *
 	 * @since 1.5.0
+	 * @since 4.5.0 Introduced the `$old_theme` parameter.
 	 *
 	 * @param string   $new_name  Name of the new theme.
 	 * @param WP_Theme $new_theme WP_Theme instance of the new theme.
+	 * @param WP_Theme $old_theme WP_Theme instance of the old theme.
 	 */
-	do_action( 'switch_theme', $new_name, $new_theme );
+	do_action( 'switch_theme', $new_name, $new_theme, $old_theme );
 }
 
 /**

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.5-alpha-36501';
+$wp_version = '4.5-alpha-36502';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.