EMI: Implement head tracking

Depends on PR #943 since this uses some of the new quaternion functionality. Once that is merged I'll rebase this.

The head tracking works fairly well, but there are some issues when head tracking is combined with animation. For example when talking to Mr. Cheese in scu his head is tilted a bit differently when compared to the original. I haven't yet seen noticeable differences with other characters though.
    engines/grim/actor.cpp: In constructor ‘Grim::Actor::Actor()’:  
    engines/grim/actor.cpp:102:23: error: cannot convert ‘std::nullptr_t’ to ‘int’ in initialization
       _lookAtActor(nullptr) {