Introduce a `.dashicons-before` CSS class.

If you want to use a Dashicon before an element you can use the class because it will not change the elements content styling. With that you don't need to copy the entire `.dashicons` styling to your :before styling anymore.
To demonstrate this change, Admin Menu now uses Dashicons classes directly.

And it fixes a glitch in IE.

fixes #26630.
Built from https://develop.svn.wordpress.org/trunk@27418


git-svn-id: http://core.svn.wordpress.org/trunk@27265 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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