Customizer: Introduce a processing state.

The processing state will prevent a save when it doesn't return 0.
Widget Customizer will use the state to prevent a save while a widget is updating, which is an asynchronous process.

props westonruter.
fixes #27390.
Built from https://develop.svn.wordpress.org/trunk@27540


git-svn-id: http://core.svn.wordpress.org/trunk@27383 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Move widget area sections to bottom.

Since a theme can have a lot of widget areas and we don't want to bury third-party sections, bump the widget sections to the bottom with a priority of 1000+. Fixes also the ordering.

props westonruter.
fixes #27401.
Built from https://develop.svn.wordpress.org/trunk@27541


git-svn-id: http://core.svn.wordpress.org/trunk@27384 1a063a9b-81f0-0310-95a4-ce76da25c4cd