Merge TransformComponent into OgreSceneNodeComponent

Now OgreSceneNodeComponent is the place for the graphics
transform, which is a natural fit for the component's purpose.

Also remove MovableComponent and MovableSystem, as they are
obsolete now.
Fix memory leaks in BulletEngine

Bullet does not take ownership of all those pointers, so wrap
them in std::unique_ptr.
Clean up headers
Remove debug functions from RigidBodyComponent
Reorder BulletEngine systems

We first want to update all changes to rigid bodies, then step
the physics engine, then update the components' states with the
new values.
Refactor Bullet / Ogre interaction

The physics transform properties have been moved into the
RigidBodyComponent. This way, nobody can forget to add a
PhysicsTransformComponent, wondering why their graphics aren't
updating.

The RigidBodyComponent can now act as its own btMotionState,
updating the position and orientation whenever necessary.
Refactor Bullet <-> Ogre conversions to be more usable

Overload the function instead of giving it different names,
so it's easier to remember.

Using "inline" saves us from having to declare unused functions
as such. At least in this case.