Customizer: Prevent double-encoding in `WP_Customize_Control`.

Control's label and description may include HTML.

fixes #29572.

git-svn-id: https://develop.svn.wordpress.org/trunk@30326 602fd350-edb4-49c9-b593-d223f7449a82
Improve keyboard accessibility on Custom Header and Custom Background screen.

props florianziegler.
fixes #29289.

git-svn-id: https://develop.svn.wordpress.org/trunk@30327 602fd350-edb4-49c9-b593-d223f7449a82
Correct `@since` tag in `count_user_posts()` docs.

Props pavelevap, DrewAPicture.
See #21364.

git-svn-id: https://develop.svn.wordpress.org/trunk@30328 602fd350-edb4-49c9-b593-d223f7449a82
Customizer: Improve ability to filter active state for widget area Customizer sections.

* Mark panels, sections, controls as active if preview explicitly indicates.
* Introduce `WP_Customize_Sidebar_Section` PHP class, and `SidebarSection` JS class.
* Move logic for determining whether a sidebar section is active from the `SidebarControl` to `SidebarSection`.

props westonruter.
fixes #30235.

git-svn-id: https://develop.svn.wordpress.org/trunk@30329 602fd350-edb4-49c9-b593-d223f7449a82
Color Picker: Allow to override `mode` and `width` values for Iris via wpColorPicker options.

props jtsternberg.
fixes #30182.

git-svn-id: https://develop.svn.wordpress.org/trunk@30330 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: Add a transparent border to a link in the footer to avoid jumpiness on hover.

props iamtakashi.
fixes #30332.

git-svn-id: https://develop.svn.wordpress.org/trunk@30331 602fd350-edb4-49c9-b593-d223f7449a82
Fix selector for the "Live Widget Previews" feature pointer.

props rzen.
fixes #30158.

git-svn-id: https://develop.svn.wordpress.org/trunk@30332 602fd350-edb4-49c9-b593-d223f7449a82