Not all widgets support displaying diagnostics

[pcdavid]

All widgets in Forms can have one or more diagnostic { id kind message }.
They are all computed on the backend, and fetched by the front (sett commonFields in FormEventFragment.ts), but only some widgets actually display them.

At the time of this writing, the ones I see with explicit support are (helperText={widget.diagnostics[0]?.message} <FormHelperText>{widget.diagnostics[0]?.message}</FormHelperText>): checkbox, date-time, list, multi-select, radio, select, text-field and (maybe) reference.