Add widget management to the customizer.

This brings in the Widget Customizer plugin: https://wordpress.org/plugins/widget-customizer/.

props westonruter, shaunandrews, michael-arestad, johnregan3, akeda, topher1kenobe, topquarky, bobbravo2, ricardocorreia. And for good measure, props westonruter.
see #27112.

Built from https://develop.svn.wordpress.org/trunk@27419


git-svn-id: http://core.svn.wordpress.org/trunk@27266 1a063a9b-81f0-0310-95a4-ce76da25c4cd