whitespace tweaks
Added GTK part of new histogram functionality
Minor whitespace tweak to match GTK side
Enhancements to Histogram plugin

- fixed eteq's updates for gtk version
- added control to set the number of bins for the histogram
- refactored Plot.py into PlotBase + Plot, Histogram and Cuts plots are now
    mixin classes.  This will make it easier to maintain updates across
    toolkits.