This refactors the whole web(kit|engine|) settings mess a bit so there's a
Web(Kit|Engine)Settings object for (non-static) settings set on a
QWeb(Engine)Settings object in Qt. Everything else is set on module-level a bit
less declaratively.
The whole inheritance mess is gone, and we can now also construct a
Web(Kit|Engine)Settings object for a given tab.
Fixes#2701