QGradient: note to unwrap 'void *dummy' in Qt 6

It was already forgotten for 5.0, let's not continue
to use Qt 4 BC hacks in Qt 6, too.

Change-Id: Ifa798115562ebd15dd9beade73a49dfbc23a208f
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
QLineEdit: mark a comment with ### Qt6

Change-Id: Ia4b37e0fd88a02f3d79be9cac99018b8bb8e10cb
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
glgen: Added support for parsing the new xml spec.

The .spec file is no longer updated thus support for gl 4.4 is impossible without an update to parse the new xml spec. The legacy parser can be used with the -l (--legacy) switch.

Task-number: QTBUG-33671
Task-number: QTBUG-40090
Change-Id: I83d9380842a16e925f6c07331ee35fe035f6baa9
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>