Fix for node menu: Show the group input/output nodes in the Input/Output categories respectively, so they can be added with the usual UI in case the user deletes them. These nodes are polled out for
non-group trees (node trees not in the bpy.data.node_groups collection) to avoid confusion. For that purpose a new optional poll function argument has been added to NodeItem, which allows selectively
polling individual items in an otherwise static list.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57073 954f8c5b-7b00-dc11-b283-0030488c597c