Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

platform/javascript/api/api.cpp

@@ -29,7 +29,7 @@
 /*************************************************************************/
 
 #include "api.h"
-#include "engine.h"
+#include "core/engine.h"
 #include "javascript_eval.h"
 
 static JavaScript *javascript_eval;