modpython: move CPyRetString to own header.

It was defined twice: in modpython.cpp, and in modpython.i
And there is a fake CPyRetString in modpython.i

Now both places which use real CPyRetString include the file instead.
Fake CPyRetString is still in modpython.i

modules/modpython/retstring.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2004-2011  See the AUTHORS file for details.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published
+ * by the Free Software Foundation.
+ */
+
+#pragma once
+
+class CPyRetString {
+public:
+	CString& s;
+	CPyRetString(CString& S) : s(S) {}
+	static PyObject* wrap(CString& S) {
+		CPyRetString* x = new CPyRetString(S);
+		return SWIG_NewInstanceObj(x, SWIG_TypeQuery("CPyRetString*"), SWIG_POINTER_OWN);
+	}
+};
+
+