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
In `wp.media.mixin`, add `ua` and a `compat` map to separate and clarify browser detection and support. Leverage Underscore list iterators in `isCompatible()`.

See #27389.


Built from https://develop.svn.wordpress.org/trunk@27542


git-svn-id: http://core.svn.wordpress.org/trunk@27385 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Make temp hooks final and add inline docs.

New hooks are `dynamic_sidebar_before`, `dynamic_sidebar_after`, `dynamic_sidebar_has_widgets ` and `is_active_sidebar`.
Remove obsolete use of hacky dynamic_sidebar hook.

props westonruter, DrewAPicture.
fixes #25368.
Built from https://develop.svn.wordpress.org/trunk@27543


git-svn-id: http://core.svn.wordpress.org/trunk@27386 1a063a9b-81f0-0310-95a4-ce76da25c4cd
MCE Views: don't (re-)render views if the format of the content is "raw" to avoid adding additional undo levels on undo/redo.

Props gcorne.
Fixes #27416.


Built from https://develop.svn.wordpress.org/trunk@27544


git-svn-id: http://core.svn.wordpress.org/trunk@27387 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE: add internal command and shortcut (Alt+Shift+X) for toggling <code>. Define a button that can be added to any toolbar as `wp_code`.  See #6113
Built from https://develop.svn.wordpress.org/trunk@27545


git-svn-id: http://core.svn.wordpress.org/trunk@27388 1a063a9b-81f0-0310-95a4-ce76da25c4cd