Use proper access modifiers and add a magic `__get()` method to `Custom_Background` and `Custom_Image_Header`.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28481


git-svn-id: http://core.svn.wordpress.org/trunk@28307 1a063a9b-81f0-0310-95a4-ce76da25c4cd
`hackificator` bails on this file because the `<meta>` isn't self-closing.

See #27881.

Built from https://develop.svn.wordpress.org/trunk@28482


git-svn-id: http://core.svn.wordpress.org/trunk@28308 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove public keyword from some JS functions. Sorry.

Props ocean90.
See #22234.


Built from https://develop.svn.wordpress.org/trunk@28483


git-svn-id: http://core.svn.wordpress.org/trunk@28309 1a063a9b-81f0-0310-95a4-ce76da25c4cd
`hackificator` bails on this file because of mixed quote styles on some HTML attributes.

See #27881.

Built from https://develop.svn.wordpress.org/trunk@28484


git-svn-id: http://core.svn.wordpress.org/trunk@28310 1a063a9b-81f0-0310-95a4-ce76da25c4cd