generator: fixicate NativeStructGen
generator: fixicate NativeStructGen

Made NativeStructGen more consistent with the way Gdk.Event
and Pango.Attribute are handled. Also increases performance
because reflection is not needed anymore to marshal these
kind of structs.

antoniusriha/gtk-sharp,orion75/gtk-sharp,orion75/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,Gankov/gtk-sharp,akrisiun/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,antoniusriha/gtk-sharp,Gankov/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,akrisiun/gtk-sharp,akrisiun/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp