Experimental code for implementing thresholding

This code tries to see how PointSets could be used to implement
thresholding.
Add some TODO notes
Make it possible to draw the threshold overlay in curr plane
Define an interface that allows Overlays to redraw

There might be better ways of doing this. But this allows an Overlay
to update its graphics in the graphic library of choice.
ThresholdOverlays now redraw when threshold is changed
Label ThresholdOverlay: looks better in Overlay Manager
Add a TODO
Add a TODO
Manage memory resources a bit better
Improve drawing speed of threshold code
Remove obsolete TODO after testing
Improve code style
Some code cleanup
Provide framework for supporting ThresholdServices
Remove toy threshold command
Note in comments that this class going away
Make Threshold and Figure Services
Begin implementing Threshold plugin
Throw appropriate events during deletions

Also while at it do some minor code reorganization
Add deleteFigure() to FigureServices and call appropriately