Expose the target scene nodes of collision response animators.  Setting the target forces an update of its last position.  This allows a setPosition() to take effect without being blocked by the collision geometry.  Added comments to this effect, a unit test, and some unrelated !node checks in all the animators' animateNode() methods.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1957 dfc29bdd-3216-0410-991c-e03cc46cb475