Fixed #17090 -- Made the API specification for `ModelAdmin.get_list_display()` more consistent with that of `ModelAdmin.list_display` by separating out the admin action check boxes business. This is backwards-incompatible for those who have been using the still-unreleased `get_list_display()` method. Thanks to Ramiro Morales for the review.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17035 bcc190cf-cafb-0310-a4f2-bffc1f526a37