Set QET version to 0.200.1 when built with Qt6

Use QT_VERSION_CHECK to detect Qt6 at compile time and report
version 0.200.1 instead of 0.100.1 in that case, so Qt6 builds
are clearly distinguishable from Qt5 builds.

sources/qetversion.cpp

@@ -23,7 +23,11 @@ namespace QetVersion
 
 	QVersionNumber currentVersion()
 	{
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+		return QVersionNumber{ 0, 200, 1 };
+#else
 		return QVersionNumber{ 0, 100, 1 };
+#endif
 	}
 
 	QString displayedVersion()