The addproperty action built the field definition array without capturing
propshowoncombobox, so the 'Show on combobox' option chosen in the property
form was silently dropped for every field; only ref and code got it (they
are hardcoded in the build-from-table path). Capture it like the other
field flags so rebuildObjectClass serializes it into the generated class.
Fixes#35076.
Co-authored-by: Laurent Destailleur <eldy@destailleur.fr>
