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