CHANGED abandon the unpredictable behaviour of the Simulation Command
dialog. You now separately add simulation tabs (which have invariant
command types once created), and the dialog edits the current tab.
Also a bunch of bug fixes to make multiple simulation plots actually
work.
Also fixes a bug so that voltages, currents and power dissipations are
only probed if the flag is set -- this keeps ngspice from throwing an
error if you probe something twice (for instance, if you have .probe
commands in text and turn off the auto-probing).