Reorganized core/ directory, it was too fatty already

-Removed FuncRef, since Callable makes it obsolete
-Removed int_types.h as its obsolete in c++11+
-Changed color names code

editor/code_editor.cpp

@@ -32,7 +32,7 @@
 
 #include "core/input/input.h"
 #include "core/os/keyboard.h"
-#include "core/string_builder.h"
+#include "core/string/string_builder.h"
 #include "editor/editor_scale.h"
 #include "editor_node.h"
 #include "editor_settings.h"