The question mark stuff doesn't work though

If it's of any value, the "?" in MS3 is got by way of:

static const QString unknownFret = QString("?");

QString StaffType::fretString(int fret, int string, bool ghost) const
      {
      if (fret == INVALID_FRET_INDEX)
            return unknownFret;

which is invoked during Note::layout() after the handling of String Number texts in my PR for "3.7"... here though in Mu4 the function called is setFretString() , but its invocation occurs beforehand in relation to this commit. Maybe a relocation of the code beforehand would fix the lack of "questionable results" ... "?" ... ?

Changing the order does not help with the ?, but it does together with another small change/addition

P.S. This stuff is mainly for now as a testing, so it's not necessarily 'worthy' of going into master branch. Like for instance, apparently capo texts "transpose" an instrument, and in that sense this PR wouldn't be too desirable. I was unaware that it functioned as a "movable transposition" in MS3 rather than like a physical capo where it disables frets below the capo. It does things like:"Fret 12 now sounds as if it were Fret 15" or something.

Scorster provided an mscz. image with "It's late, and I'll have to think this through. But I don't think a Staff Text Line with Capo properties needs to affect anything in the tablature staff":

The capo 5 and 8 still allow for below capo fret positions. Not sure how to continue with it. For me personally, it would've made sense to disable frets below the capo and then reform the automatic translation from treble to linked tab score if possible, but that's even more work than what has been done so far.... as it would require not just updating any one fret but the entire automatically formed chord shape....

So maybe should retract that second commit so that it continues as it was designed by whoever designed it.

Even if it currently is just for testing and work in progress: testing and comments are welcome