[1.2.X] Fixed #13095 -- `formfield_callback` keyword argument is now more sane...

[1.2.X] Fixed #13095 -- `formfield_callback` keyword argument is now more sane and works with widgets defined in `ModelForm.Meta.widgets`.  Thanks, hvdklauw for bug report, vung for initial patch, and carljm for review.  Backport of r13730 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13731 bcc190cf-cafb-0310-a4f2-bffc1f526a37