Disentangled some parts of the admin ChangeList and ListFilter's internals. With this refactoring, the query string lookups are now processed once instead of twice and some bugs (in particular the SimpleListFilter parameter name being mistaken for a model field in some cases) are avoided.

      Refs #17091.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17145 bcc190cf-cafb-0310-a4f2-bffc1f526a37