Resurrect tool that binds ThresholdOverlays and JHotDraw

Note that this tool is required to correctly create and tie together
JHotDraw threshold figures and ThresholdOverlays. This implementation
specifies the tool as invisible so that users can't invoke it. It is
invoked by the general overlay creation/view code.
Delete FigureService code

This code was hatched to create JHotDraw figures for thresholds. Note
that the SwingThresholdTool now takes care of this. The new approach
is more reliable and thus these classes can go away.