Check for data types like gint and double, and for NULL function results in case of errors

[StefanSalewski]

First, from https://wiki.gnome.org/Projects/GObjectIntrospection/Annotations

(nullable)

Indicates that NULL may be a valid value for a parameter (in, out, inout), or return value (though note that return values which are only NULL when throwing an error should not be annotated as (nullable)).

So we have to catch and handle NULL results for all functions, not only when g_arg_info_may_be_null(). Maybe check GI_FUNCTION_THROWS.

Second, we should check if gint is replaced by int32 or cint. cint seems to be correct. And GTK's gdouble should be always converted to Nim's float, even when it is identical currently.

And finally, someone should test on a 32-bit system and on Windows or Mac.