Changes made to the data used to populate Menu.fields isn't picked up by Menu.display

[nmlorg]

Breaking this off from #63-511009850:

This also exposes the general issue that whenever menu.fields changes (either by having a field added/removed during processing, or by having its desc change), that change does not get picked up automatically if/when we call menu.display. The latter (desc changing) might suggest we should have desc be a callable, to both skip calculation if it's unnecessary (if we enter a msg.action-setting handler) and defer it (if it is necessary) until any handlers/local logic have made changes to the data. The former (fields being added/removed), however, goes back to suggesting that fields itself should be callable, and/or some kind of reusable generator.