GDLVL/GMD conversion support & Migrate to PySide6

BUILDING.md

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+# macOS
+
+nuitka --standalone --enable-plugin=pyside6 --macos-create-app-bundle --output-dir=build GDUtils.py
+
+# Windows
+
+nuitka --standalone --enable-plugin=pyside6 --windows-console-mode=disable --output-dir=build GDUtils.py
+
+# Linux
+
+nuitka --standalone --enable-plugin=pyside6 --output-dir=build GDUtils.py
+