Add support for T/t and P/p in fretboard fingerings (Fixes #23755)

[pyonpyoco]

Resolves: #23755

Description:
This PR adds support for entering T/t (thumb) and P/p (pulgar) as fingering values for fretboard diagrams.

Changes:

• Updated DOM fingering parsing and internal values.
• Mapped T/t → 6, P/p → 7 internally to keep the existing data structure.
• Updated Inspector UI to allow entering T and P.
• Updated reading/writing logic to support these values.

Backward compatibility:
Opening scores using this feature in older versions (4.6.3) of MuseScore currently displays the digits 6 and 7.
Please advise if backward compatibility handling or a conversion layer is preferred.

UI example

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[avvvvve]

@pyonpyoco Thanks for working on this. Would you mind rebasing so I can test this?

As to mapping the letters to 6 and 7 in older versions, I think it would be better if we just wiped the field clear if they contained T/t/P/p. 6 and 7 don't map to any fingers after all, so it would be better to erase the marking entirely.

[pyonpyoco]

@avvvvve Thanks, I’ve rebased onto the latest master so you can test it.

I’ll take another look at the backward-compatibility behavior you suggested.

Just to confirm: do you mean clearing only the unsupported T/t/P/p fingering entries for backward compatibility, while keeping valid numeric fingerings unchanged?