Fix #17185: MusicXML ignores custom string data

[HemantAntony]

Resolves: #17185

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[HemantAntony]

This PR also solves another unreported problem. When you tune a string to a -1 octave, say C-1 and export it to MusicXML, the exported file gets corrupted because the minInclusive value of tuning-octave in the schema is 0

[Jojo-Schmitz]

I don't quite understand how the changes of this PR could lead to those unit test failures, which are about the number of frets but not their tuning? But I guess just adjusting that test is the correct way of dealing with this

[lvinken]

A quick check shows at least the staff tuning is imported correctly with this PR.

Please also add one or more test files.

[DmitryArefiev]

#17185 also looks fine on my side (Windows10) , but tests should be fixed at first. @HemantAntony Can you look into it please?

[Jojo-Schmitz]

That mtest failure is related to the 1st commit, not to the 2nd

[HemantAntony]

Fixed the tests and updated the PR. I also added two tests to test both problems (#17185 and negative octave).

Also should I add PDFs of the test files? I see there are PDFs of xml and mscx

[Jojo-Schmitz]

This seems to be C++17 and later only, am I right that octave = std::max(std::min(octave, 9), 0); is the same thing for older C++ standards?

[DmitryArefiev]

Tested #17185 on Win10, Mac13 - FIXED

@HemantAntony Thanks for fixing!