Customizer: When navigating around the site within the Customizer preview, update the document title.

props westonruter.
fixes #28542.
Built from https://develop.svn.wordpress.org/trunk@30306


git-svn-id: http://core.svn.wordpress.org/trunk@30305 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Use `jQuery.fn.toggle()` instead of `slideUp`/`slideDown` if panel/section/control is not inserted into DOM yet.

jQuery does nothing when calling `slideUp` on elements that are not inserted into the DOM yet, which can now be the case now when first loading the Customizer as the panels, sections and controls get dynamically inserted, see #28709.

props westonruter.
fixes #30251.
Built from https://develop.svn.wordpress.org/trunk@30307


git-svn-id: http://core.svn.wordpress.org/trunk@30306 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Trigger `widget-added` event when initializing.

Widget controls are now added to the pane dynamically via JavaScript, see #28709.
Remove the event trigger from `SidebarControl.addWidget()` as it's covered by `WidgetControl.ready()`.

props westonruter.
fixes #30236.
Built from https://develop.svn.wordpress.org/trunk@30308


git-svn-id: http://core.svn.wordpress.org/trunk@30307 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Add support for the media library in Background Image controls.

`WP_Customize_Upload_Control` (PHP) and `api.UploadControl` (JS) can now open the media modal.

props celloexpressions, and ericlewis for docs.
see #21483.
Built from https://develop.svn.wordpress.org/trunk@30309


git-svn-id: http://core.svn.wordpress.org/trunk@30308 1a063a9b-81f0-0310-95a4-ce76da25c4cd