Context menu is now selection-dependent.  Also added option to compare
two traces side-by-side if two are selected
Tweak cluster visualization context menu
Added a method to calculate how volatile each node is in the graph
Renamed cluster viz
Moved report hashing functions to xtrace_utils
Tweaked a few things:
- Gotta remember to remove those hidden 'Operation' nodes
- Added function on the graph to relabel an individual node
- Overloaded the 'remove' method to allow either a node id or a node as
an argument.  Do a string type check to start with
Fixed some major bugs whereby I should have been iterating over an
original graph, modifying a copy of it.  Beforehand I was iterating over
the copy, which meant the stuff I was iterating over was changing as I
was iterating over it.

In addition, I've added a 'calculate_node_stability' method which is
used in the graph comparison visualization to highlight the nodes that
are the 'volatile' nodes.  I've put in a first pass at this
visualization, but there are a couple of things to clean up.  In
addition, it is currently uni-directional in its calculation of WL
labels, whereas the actual kernel should be bidirectional.  I will fix
this shortly.