Add additional side views for proper 3D editing

[Schmackbolzen]

It should be fairly easy to add three additional views (xy,xz,yz plane) like many proper 3D editors have. This will make 3D editing a lot better.
Here is an example how multiple views can be added to the wpf renderer:
https://stackoverflow.com/questions/45204921/multiple-viewport-with-same-model-in-wpf
The layout should be a 2x2 grid where 4. view is the old 3D view. You can look at e.g. Valve's Hammer Editor if you want an example. It also might be helpful to only draw in wireframe mode there, but this has to be tested (or made an option).