Revisions UI update:

 * Style status loading as a regular update message like on the other screens
 * Move compare two checkbox to diff header to avoid an overlap on long post titles
 * Restore color shades for the scope of changes visualisation 
 * Show tooltips while moving the slider handle
 * Diff header styling
 * Move color declarations to classic/fresh stylesheets
 * Sync admin color stylesheets

props jrbeilke for initial patch. see #23935.

git-svn-id: https://core.svn.wordpress.org/trunk@24119 1a063a9b-81f0-0310-95a4-ce76da25c4cd