Fix #35633, Cannot Add Group Node In Blender 2.67a. The menu entry for the "make group" operator was missing in the new categories system. Added an alternative NodeItemCustom to the standard NodeItem to
allow custom draw functions such as this operator. Used in the group items callback to generate the basic group_make operator call.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57249 954f8c5b-7b00-dc11-b283-0030488c597c