Fix builds with Qt 6

[Tomin1]

This fixes building with Qt 6 and removes all deprecation warnings as a bonus.

Main things to adapt:

• A few places like QVariant and QDBusMetaType no longer accepted int but wanted QMetaType. That had to be ifdef'd because those versions of the methods were only introduced in Qt 6.
• Forward declarations of Qt types are forbidden in some places.
• Opaque pointers as property types need to be declared.
• QVariant::type() is deprecated while QVariant::userType() is fine to use still.

[pvuorela]

Small naming comment, but I'll just approve already. I'm ok with either, or some new alternative.