Port to Qt6
Port to Qt6/QtWebEngine6: the build can be enabled with -DQT_MAJOR_VERSION="6". Should still build and run fine with Qt5 by default. Mostly fixing deprecated calls. Includes all extensions, including Python (using PySide6) and KDE Frameworks extension (using the latest git HEADs which are Qt6-based).
It does need testing, as I haven't been able to test it extensively. It does build and run, and I haven't had any issues with any websites in my admittedly limited testing.