Revise menu width handling via a new trait.

Revise menu width handling via a new trait.

get_widths has become check_widths, which subsumes the processing done
at every call site, and will be easier to extend to the next thing I
have in mind.

Also, the three width-aligning methods for menu lines are now part of
the MenuKeypressLineGeneral trait. That only has one implementing type
right now, but another one will be along shortly.