fix #42851: issues setting SHOW_COURTESY for courtesy signatures

The SHOW_COURTESY property really only applies to "main" key and time signatures, not to the corresponding courtesy signatures themselves.  Attempting to set this property via Inspector for courtesy signatures was leading to problems as described in referenced issue.  Fixed to disable the Inspector checkbox for generated key signatures when selected individually, but also changed setProperty to do nothing for generated key signatures, so the checkbox can still work for non-generated signatures when multiple signatures are selected.