QQmlPropertyBinding: support interceptors

QQmlPropertyValueInterceptor currently interacts by intercepting the write metacall (using QQmlInterceptorMetaObject::intercept).
QQmlPropertyBinding does however bypass the interceptor, as it doesn't do any metacall at all, but directly writes to the property. This obviously breaks interceptors like Behavior.

Ideally we implement this without having to create and go through some custom QQmlVmeMetaObject; rather, we should give QQmlPropertyBinding a hook which QQmlPropertyValueInterceptor can use (with potentially new API). This is possible as interceptors are private API.