Allow Launcher To Set Factory

[msteinbeck]

I'm working on #752 (currently it is for my project only but maybe I can make a PR of it later) and found that creating a second, partial model is not that easy due to the fact that both models (and all created elements) use different factories which, if I am right, yields to errors while resolving references. It would be nice if one could set the actual factory of a launcher before building the model. This way, I could reuse the factory of my main model and ensure that both, my old and new types, share the same factory.