Simplified the admin changelist multi-sort interface specifically by removing the popup window, adding explicit tooltip help texts, improving the hover visual states and allowing all operations (i.e. removing a column from sorting and toggling the sorting with and without changing the sorting priority) to be actionable with just one click. Many thanks to Idan Gazit for the feedback and direction. Refs #16212.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16899 bcc190cf-cafb-0310-a4f2-bffc1f526a37