Move Windows build to 64-bit PySide6 to resolve high-DPI/scaling issue (fixes #793 by SquirrelKiev and #707 by ritiek)

[Et0h]

Adds a new 64-bit Windows build using Python 3.11 and PySide6, resolving the Windows high-DPI/scaling issue reported in #793 by @SquirrelKiev.

As part of this work I also resolves the same issue that @ritiek was trying to fix in #707 about the invocation of the font dialogue being broken in PySide6. The holding issue on that PR was that the change didn't work on PySide2.

The existing 32-bit Python 3.8 / PySide2 build is retained as a fallback for now, but might be removed in the future.

Summary

• Add a parallel Windows x64 build using:
  • Python 3.11
  • PySide6
  • py2exe 0.14.0.0
• Keep the existing Windows x86 / PySide2 build unchanged.
• Update buildPy2exe.py to support packaging either PySide2 or PySide6.
• Package the Qt6 Windows platform and modern Windows style plugins.
• Use Qt6's native high-DPI handling rather than applying the old Qt5 high-DPI attributes.
• Fix PySide6 handling of persisted QFont weights (resolving the issue of Fix invocation of get font dialog #707 without the compatability issues).
• Fix Windows/Qt6 window flags for the main window and custom dialogs.
• Prune unused Qt6 libraries from the packaged build.
• Add a packaged x64 GUI startup smoke test to the Windows build workflow.
• Fix generated Windows uninstaller directory removal.
• For fresh x64 installations, default to the native 64-bit Program Files directory.
• When upgrading an existing installation, continue using its registered installation directory

[SquirrelKiev]

should fix #611 too