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.


git-svn-id: https://develop.svn.wordpress.org/trunk@27419 602fd350-edb4-49c9-b593-d223f7449a82