Design 3D models

[Indy2222]

The goal is to come up with first design of 3D models (and their textures and so on). The models should have consistent style with one another and with other aspects of the game.

The list and description of all buildings and units is available in docs.de-game.org/design/.

How to contribute models is documented here.

The models should be accompanied by object files (#276). @Indy2222 can help with that.