WP Admin jQuery UI

As proposed in Trac ticket #18909, there should be some sort of matching CSS for the WordPress admin now that all jQuery UI components are bundled. While decisions would have to be made about how these are bundled, if at all, this is at least a start to having [a] matching stylesheet[s] for the WordPress admin and jQuery UI.

This is a plugin that will create a jQuery UI demo page under the Tools menu. Color scheme will change based on the user's setting. Scripts and stylesheets are enqueued on the demo screen only.