Add door/window placement preview and calculate the model positions properly
Similar to object model previews, now the outlines of new doors/windows are previewed using the proper model when placing them.
As a nice side-effect this also adds the model placement code to the model manager which will be used to position the models after they have been placed.
![](https://olives.aztwantie.de/wp-content/uploads/2019/11/image-3-1024x714.png)