Refactor QVariant internals

QVariant currently still uses it's own duplicated code to handle built-in types.

The handler struct in QVariant, including the gui and widget handlers should get removed, and we should use the handlers provided by QMetaType instead.

This will cause our built-in and external types to use the same code paths, and should simplify a lot of the QVariant related code.

Open item here are the automatic conversions QVariant does. These are not reflected in the QMetaType data, and probably should not be handled there neither. So this might still require some special handling, but it should not go through the current handler structs.