Add 'IObjectMarker' to universally identify markers.

Triggers are usually defined by IObjectTrigger, yet the rest of the markers are just left as IObjects. This would help simplify scripts that try to filter out these kinds of objects which are not visible nor interactable. The included objects are pretty much ALL of the joints, perhaps the zones as well, specific ones such as DestroyTargets & WeldJoint and so on.