A fix for functions returning wxFont objects

I've added a function "is_ref_counted_class" where we can selectively enable the fix to issue #17 for specific classes. In this commit I have only enabled it for wxFont. If it works well we can enable for more classes.

For me, this fix seems to compile and run well. You could try reverting to commit dc524058b105eb5cd737baec93b13d41c89e356a and then applying this one.