yacas-gui Qt6 port

[a17r]

Are there any plans, or work underway already, to port to Qt6? 6.7.2 is the current version, so this is very far into a mature release phase, and real Qt5 OSS support stopped back in 2020 already.

That means distributions are actively going to start rooting out - at first - Qt5WebEngine revdeps, of which yacas-gui is one, since this is on its last legs and a real burden to maintain. Without Qt5WebEngine it could survive much longer in distributions' respositories, but still the need to port to Qt6 is there.