Request: Qt WebEngine 6 support

[hijackeel]

Firstly, thanks for this fork; I've been using it for two years.
Gentoo dropped Qt WebEngine 5 back in April though, and this is the only software on my system still requiring it.
It would be nice to have an update supporting Qt WebEngine 6.

[vedgy]

It makes sense to wait for a Qt WebEngine port reviewer and merge the branch into master first. Otherwise, I'll have to maintain and fix merge conflicts in further divergent branch(es) myself. And minor releases of Qt 6 will likely cause regressions to adapt to.

Most distros still package Qt 5 WebEngine. And Gentoo users compile from source anyway, so building the not-officially-supported Qt 5 WebEngine shouldn't be a big hurdle.