Optional "atlas box" definition in model definitions

Currently when a model is scaled to fit with the atlas bbox it's done against the Ogre bounding box. However, in some cases this can lead to very inexact scaling, which is noticable for example with entities that we want to fit precisely against each others (such as wall segments).
We should therefore add an ability to in the model definition specify an "atlas box", which is the bounding box which it should try to fit the server entity against. This should be doable in the model edit widget.