Extract ScriptInstance to simplify includes

This allows to include script_instance.h directly in the
generated gdvirtual.gen.inc, and remove excessive includes
from the codebase.

This should also allow Resource to use GDVIRTUAL macros,
which wasn't possible previously due to a circular dependency.

core/object/make_virtuals.py

@@ -160,6 +160,8 @@ def run(target, source, env):
 #ifndef GDVIRTUAL_GEN_H
 #define GDVIRTUAL_GEN_H
 
+#include "core/object/script_instance.h"
+
 
 """