Translate Model class' storage to C++.

Some code copied from xs-model branch.

Also:
* Generate ::Ref classes programatically.
* Add separate __REGISTER_CLASS macro
    (for use where forward declaration won't work, i.e. typedefs)

xs/src/StringMap.cpp

@@ -0,0 +1,13 @@
+#include "StringMap.hpp"
+#ifdef SLIC3RXS
+#include "perlglue.hpp"
+#endif
+
+
+namespace Slic3r {
+
+#ifdef SLIC3RXS
+__REGISTER_CLASS(StringMap, "StringMap");
+#endif
+
+}