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

props westonruter.
fixes #28542.

git-svn-id: https://develop.svn.wordpress.org/trunk@30306 602fd350-edb4-49c9-b593-d223f7449a82
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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30307 602fd350-edb4-49c9-b593-d223f7449a82
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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30308 602fd350-edb4-49c9-b593-d223f7449a82