NodeGraph Widget doesn't group by namespaces

[MJohnson459]

rqt_graph groups nodes by namespace to make it easier to see connections between groups of components.

Some sort of equivalent grouping/simplifying is required for this widget to be useful for large projects.

http://wiki.ros.org/rqt_graph?action=AttachFile&do=get&target=snap_rqt_graph_moveit_demo.png