Coding Standards: Move wp-admin/custom-header.php to `wp-admin/incl…

…udes/class-custom-image-header.php`

This renames the file containing the `Custom_Image_Header` class to conform to the coding standards.

This commit also includes:
- A new `custom-header.php` that includes the new file, for anyone that may've been including the file directly.
- Replaces references to the old filename with the new filename.

See #47632.



git-svn-id: https://develop.svn.wordpress.org/trunk@45654 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/custom-background.php

@@ -518,7 +518,7 @@ public function handle_upload() {
 		$thumbnail = wp_get_attachment_image_src( $id, 'thumbnail' );
 		set_theme_mod( 'background_image_thumb', esc_url_raw( $thumbnail[0] ) );
 
-		/** This action is documented in wp-admin/custom-header.php */
+		/** This action is documented in wp-admin/includes/class-custom-image-header.php */
 		do_action( 'wp_create_file_in_uploads', $file, $id ); // For replication
 		$this->updated = true;
 	}