Customizer: Add `_wp_attachment_is_custom_background` meta to uploaded background images.

Adds `$type` property to `WP_Customize_Background_Image_Control` (PHP) and introduces `wp.customize.BackgroundControl` (JS).

see #30707.
Built from https://develop.svn.wordpress.org/trunk@30885


git-svn-id: http://core.svn.wordpress.org/trunk@30873 1a063a9b-81f0-0310-95a4-ce76da25c4cd