Fixed #16059 -- Improved the usability of the admin's vertical and horizontal...

Fixed #16059 -- Improved the usability of the admin's vertical and horizontal "filter" widgets, in particular by providing a better visual representation of the buttons' enabled and disabled states, and by providing more elaborate, yet less cluttered, help texts.

Note that this commit is an exception to the current tacit rule that javascript code changes should be avoided until a proper javascript testing framework for Django core is in place. This exception is commanded by the fact that it is to fix a recognized usability issue and that the patch has been (manually) extensively tested in IE6+, Chrome, Safari, Firefox and Opera.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16714 bcc190cf-cafb-0310-a4f2-bffc1f526a37