Add "step through" trace for Stochastic Character Mapping

[jcoliver]

If there is not a way to export a summary of an arbitrary number of SCM traces (on an arbitrary number of trees), this could be useful. This isn't necessarily a number for tree and matrix (more like a table for tree and matrix), but would show the total number of each type of change. The output would be a text file of the sort:

start	end	times
0	1	3
0	2	2
1	0	1
1	2	5
2	0	0
2	1	1

This information is generated for each trace (accessible via Trace > Show Difference Window), but an option to export a summary for multiple traces (like 1000 or more) would be nice functionality to have via the GUI. (although perhaps this GUI-based functionality already exists?).