Fixed #11434 -- Corrected style of arrow buttons located at the center of...

Fixed #11434 -- Corrected style of arrow buttons located at the center of filter_{horizontal|vertical} m2m widgets to not give them focus borders that span from the left page border. Thanks defaultwombat for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15144 bcc190cf-cafb-0310-a4f2-bffc1f526a37