Refactor the model system

The current model system works really well, but it currently hinges on a Model being a subclass of Ogre::MovableObject.
This has some advantages, mainly in easy of use, but some serious disadvantages in performance. The methods on Ogre::MovableObject are called very frequently each frame, and the way Model needs to iterate through containers each time brings bad performance.

Therefore we should look into refactoring it into a different structure where Model isn't acting as an instance of Ogre::MovableObject but instead as an orchestrating class, managing and handling the various entities etc. which make up the model.