Reorganized artifact-related classes

- files now generally contain only 1 class (except for tightly coupled
classes)
- files are now located in lib/entities/artifact directory
- removed excessive includes

No changes to functionality

lib/mapObjects/IMarket.cpp

@@ -11,12 +11,12 @@
 #include "StdInc.h"
 #include "IMarket.h"
 
-#include "CArtHandler.h"
 #include "CCreatureHandler.h"
 #include "CGObjectInstance.h"
 #include "CObjectHandler.h"
 
 #include "../GameLibrary.h"
+#include "../entities/artifact/CArtHandler.h"
 
 VCMI_LIB_NAMESPACE_BEGIN