GUI should start immediately, without waiting for model to be loaded

[hannobraun]

Right now, when starting the Fornjot app, first the model is being loaded, then the GUI is shown with the loaded model. This causes problems:

• Sometimes loading the model takes longer, for some reason. Then it takes a while for the GUI to show.
• If there's an error loading the model (for example a compile error in the model code), the GUI doesn't load at all.

This is very confusing. I think the solution is to load the GUI first (not showing any model), then show the model once it is ready. I don't think this will be hard to achieve, but currently code in fj-window/fj-viewer assumes that there is always a model, so this would have to be adapted.

I'm classifying this as https://github.com/hannobraun/Fornjot/labels/type%3A%20bug, since this is such a confusing problem. Especially if the model doesn't load at all, since there is no GUI to show the error then.