Improve QScriptClass API for prototype usage

Currently it's not possible to use QScriptClass to create a "fallback" object in the prototype chain, i.e. one that handles any property access if it's not handled by the instance ("this") itself.

If you put a QScriptClass-based object in the prototype chain, the object argument received in e.g. the queryProperty() reimplementation will be the QScriptClass instance, not the this-object. This is confusing; the methods should be named queryOwnProperty() and ownProperty().

The QScriptable class provides the this-object, but it's specific to QObjects. A general-purpose solution for being able to handle property access through a prototype object would be nice.