Coding Standards: Move wp-admin/custom-background.php to wp-admin/includes/class-custom-background.php

This renames the file containing the Custom_Background class to conform to the coding standards.

This commit also includes:

- A new custom-background.php that includes the new file, for anyone that may've been including the file directly.
- Replaces a reference to the old filename with the new filename.

See #47632.

Built from https://develop.svn.wordpress.org/trunk@45662


git-svn-id: http://core.svn.wordpress.org/trunk@45473 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -2617,7 +2617,7 @@ function _custom_header_background_just_in_time() {
 		add_action( 'wp_head', $args[0]['wp-head-callback'] );
 
 		if ( is_admin() ) {
-			require_once( ABSPATH . 'wp-admin/custom-background.php' );
+			require_once( ABSPATH . 'wp-admin/includes/class-custom-background.php' );
 			$custom_background = new Custom_Background( $args[0]['admin-head-callback'], $args[0]['admin-preview-callback'] );
 		}
 	}

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45661';
+$wp_version = '5.3-alpha-45662';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.