Change CMake code to enable building against Qt 5 or 6
- forward declaring QStringList doesn't work with Qt6, instead #include it (for Qt6 we could use 'using QStringList = QList' but that wouldn't work with Qt5, and it's not worth an #ifdef, QStringList is common in Qt and KDE code anyway)
- Add a copy constructor to PolkitQt1::Details, required because of QMetaType and the QESDP d-pointer
By default this builds with Qt5, to build with Qt6 pass -DQT_MAJOR_VERSION=6 to cmake.