This essentially reverts commit 894931a and then inlines `try_set` in
the one place where it's used. If we ever want to reuse that code, we'll
factor it out again.
Invalid attempts to specify a regulator's set point are now local to
each view of the regulator, and don't affect the regulator model. In
particular, a regulator will be valid and in force even when one of its
regulator input views is showing an invalid specification attempt. The
invalidity indicator should therefore be tied to the input showing
the invalid specification, not to the whole regulator outline item.
Make a regulator's set point specification private, and split the set
point into a private writable signal and a public read-only signal. The
set point can now be initialized only through the factory method
`insert_new_regulator` and changed only through the setter method
`try_specify_set_point`, which both ensure that the set point
specification is valid and consistent with the set point.
Distinguish constraints from observables using dark background rather
than marker. Customize focus highlighting. Drop the input type selector
that used to make styling apply to the Lorentz product field but not the
constraint activation check box.
