Deprecate add_custom_image_header(), remove_custom_image_header(),...

Deprecate add_custom_image_header(), remove_custom_image_header(), add_custom_background(), remove_custom_background(). Replacements are add_theme_support() and remove_theme_support(). see #20249.

git-svn-id: https://core.svn.wordpress.org/trunk@20218 1a063a9b-81f0-0310-95a4-ce76da25c4cd