Node Groups Editor Tour

Showing a couple of new node group editing features and changes. Overall design ideas and discussion can be found here: http://code.blender.org/index.php/2012/01/improving-node-group-interface-editing/ and here: http://blenderartists.org/forum/showthread.php?245047-Node-Groups-test-builds Code for interface nodes is uploaded for review here: http://codereview.appspot.com/5593050/ Note that this patch only includes the interface nodes for now, not the groups-inside-groups editing. This might go in as a separate patch, depending on how well received it is.