Handle Qt 6.7 changes to QKeyMapper::possibleKeys, with backwards compatibility

[ferdnyc]

Resolves: #23980

Qt 6.7 changes the return type of private QKeyMapper::possibleKeys from QList<int> to QList<QKeyCombination>. A QKeyCombination can easily be transformed into an int by calling its .toCombined() method.

Since MuseScore really wants a QSet<int>, the return value must be transformed regardless. So, to avoid ugly ifdefs, I use auto for the variable that accepts the return from possibleKeys() (which will be either QList<int> or QList<QKeyCombination>), then call a newly-added function convertToSet() which has two overloads: one takes a QList<QKeyCombination>, the other a QList<int>, and both return QSet<int>. The correct one will be called based on what the current version of Qt returns from QKeyMapper::possibleKeys().

This makes the code buildable with Qt 6.7, without affecting its compatibility with earlier versions.

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[ferdnyc]

(While this does get MuseScore to build successfully with Qt 6.7.2, it doesn't solve the UI issues that remain when running the application. See the discussion in the linked issue for more details.)