Harmonise colour/style of our alert boxes with jQuery UI

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5172 2fea6a4d-a838-0410-917b-93a53c48e9d2
Bit of visual feedback on active sortables

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5173 2fea6a4d-a838-0410-917b-93a53c48e9d2
Stamp on jQuery UIs use of classnames for curvy corners

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5174 2fea6a4d-a838-0410-917b-93a53c48e9d2
Merge remote-tracking branch 'upstream/master' into jquery-ui-integration