Expose getPointedThing to Lua

This commit introduces Raycast, a Lua user object, which can be
used to perform a raycast on the map. The ray is continuable, so one can
also get hidden nodes (for example to see trough glass).

src/script/cpp_api/s_item.h

@@ -52,6 +52,7 @@ public:
 protected:
 	friend class LuaItemStack;
 	friend class ModApiItemMod;
+	friend class LuaRaycast;
 
 	bool getItemCallback(const char *name, const char *callbackname);
 	void pushPointedThing(const PointedThing& pointed);