Customizer: Use deep-links for Backgrounds, Headers, and Widgets.

Replace links in admin menu and toolbar to Custom Background/Header screen with deep-links to the Customizer section.
On the Widgets screen display a link to the Customizer widgets panel.

props topher1kenobe, rzen, celloexpressions, westonruter
fixes #25569, #25571, #28032.

git-svn-id: https://develop.svn.wordpress.org/trunk@30459 602fd350-edb4-49c9-b593-d223f7449a82