"Show courtesy" checkbox in clef Inspector does not work

Well, it works if you have the courtesy clef itself selected, but not with the main clef. Technically, I know the apparent courtesy is the main clef, but anyhow - I mean it doesn't work the way people expect, where you select the clef at the start of the system then toggle the option in Inspector.

I broke this in the lamest possible way: the code in the Inspector responsible for sorting out which clef is which does this only for the show courtesy option, which it checks for by testing if "idx == 6". When I added a new field for autoplace min distance, that idx became 7. Incredibly, that code worked for 6 years. In all fairness, I don't think the current property system that makes this kind of thing rather easier to check now existed then.

I will submit a PR.