Misc fixes and cleanups; fixed variadic Lua calls to self.

Script interface:
- entity_msg() will no longer corrupt the Lua stack if sending messages to itself.
- added more info to non-critical Lua errors ("attempt to call a nil value", etc)
- replaced many lua_tostring() with getString(), which does never return NULL.
  This prevents possible crashes when a non-string parameter is passed to functions
  expecting a string.

Misc:
- Removed classes BoxElement, DFSprite, Datafile, and related references.
  They were essentially unused.
- Removed unused Element class member variables.
- Show more lines in the in-game console.

BBGE/SkeletalSprite.cpp

@@ -740,7 +740,7 @@ void SkeletalSprite::animate(const std::string &animation, int loop, int layer)
 	animLayers[layer].animate(animation, loop);
 }
 
-float SkeletalSprite::transitionAnimate(std::string anim, float time, int loop, int layer)
+float SkeletalSprite::transitionAnimate(const std::string& anim, float time, int loop, int layer)
 {
 	AnimationLayer *animLayer = getAnimationLayer(layer);
 	if (animLayer)

BBGE/SkeletalSprite.h

@@ -217,7 +217,7 @@ public:
 	void stopAnimation(int layer=0);
 	void stopAllAnimations();
 
-	float transitionAnimate(std::string anim, float time, int loop=0, int layer=0);
+	float transitionAnimate(const std::string& anim, float time, int loop=0, int layer=0);
 
 	bool isAnimating(int layer=0);