qmltc: Provide equivalent of QQmlComponent::createWithInitialProperties()

Currently, qmltc-compiled classes can only be initialized with default values. We need a way to specify initial property values, so that they are available by the time Component.onCompleted is run.

Some ideas:

Candidate 1

A manual completion interface, e.g. an equivalent of QQmlComponent::beginCreate() + QQmlComponent::beginCreate()
 

Candidate 2

A constructor or static member function which takes callback to initialize properties before completion, for instance:

template<typename Fn>
GeneratedTypeConstructor(QQmlEngine* engine, Fn &&initializer, QObject* parent = nullptr) :
    GeneratedTypeConstructor{parent}
{
    initializer(*this);
    complete(engine);
}

// ...
new GeneratedTypeConstructor(&engine, [&](GeneratedTypeConstructor &o) { /*set properties of o*/ }, parent);

A callback is suggested here instead of QVariantMap to support strong typing.