(Particularly useful for those that have no other settings and
otherwise look blank.)
Also replaces "Generator" terminology as it's a bit confusing
(and will only get more confusing with PCBNew Generators).
On Windows, ngspice doesn't output messages to stdout or stderr, it
starts the gui which causes KiCad to hang until ngspice is terminated.
We want to handle immediate responses but not get hung by the external
process. Passing a process class to wxExecute allows us to detach the
process (it will terminate and free itself) while still handling the
immediate output and errors
Fixes https://gitlab.com/kicad/code/kicad/-/issues/15446
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).