Consider removing the additional Qt5 namespace in Qt5Compat

Please consider removing the additional Qt5 namespace in Qt5Compat. This requires projects using classes from there not only to touch the buildsystem to link to the module but also the code itself. This hampers migration from Qt5 to Qt6.

Note that adding a plain 'using namespace Qt5;' in some central header is /not/ sufficient as a work around when using QTextCodec.