This allows Qt Quick Designer to register a component that can serve
as a mockup for known C++ components registered in e.g. main.cpp.

In many cases those components are the interface to the C++ backend.
While the C++ components itself are not relevant for the gui designer,
the user has to be able to instantiate gui components that use such C++
components.

We use the CreateSceneCommand to forward a list of C++ types to
qml2puppet.
Those types are then registered so that the imports and object institation
works.

Change-Id: I1543912f233f9a783998f3c6a1b48981b342ee80
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>