Fix #302114: Wrong default GUI font under Windows

[Spire42]

Resolves: #302114

Worked around a problem in Qt that caused MuseScore to use the wrong default GUI font on Windows. Qt 5.x and below use the deprecated function GetStockObject() with DEFAULT_GUI_FONT, which returns MS Shell Dlg 2 in 8 pt. MS Shell Dlg 2 is a virtual font that maps to Tahoma, which has not been the default Windows GUI font since 2006.

The correct way to determine the default GUI font is to call SystemParametersInfoW() with SPI_GETNONCLIENTMETRICS and use the returned lfMessageFont structure. On all versions of Windows from Windows Vista through Windows 10, this typically returns Segoe UI in 9 pt.

This problem is slated to be fixed in Qt 6. For details, see QTBUG-58610.

In the meantime, we can work around the problem by having Qt use the "QMessageBox" font instead, which is already being initialized the correct way.

There are two parts to this fix:

1. Override Qt's detection of the default GUI font. To do this, we ask Qt for the "QMessageBox" font and then tell Qt to use that as the default GUI font as well.

2. Detect existing settings files that have been saved with the incorrect default GUI font, and reset the incorrect font settings so that the correct font can be picked up automatically by existing MuseScore installations. Note that this will have the side effect of making the MS Shell Dlg 2 font no longer “stick” if the user explicitly selects it. However, this is a virtual placeholder font that shouldn't be explicitly selected anyway, and any users who prefer its look can always explicitly select the actual underlying font, Tahoma.

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[Spire42]

I did some more digging and I found a better workaround.

It turns out that there's already an existing code path in the Qt codebase that's initializing fonts the correct way; specifically, it's the code in QWindowsTheme::refreshFonts() that initializes the default fonts for specific classes, such as QMessageBox. So all we need to do is ask Qt for the "QMessageBox" font and then tell Qt to use that as the default GUI font as well. This has the exact same result as doing it manually ourselves.

I've just updated the code in this PR with this better workaround.

[Spire42]

Refiled as #5820.