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.
The pointer from native is stored inside of a class which
wraps the structure. Fields can be accessed by marshalling
from and to the pointer. glib: Value.Update does now invoke
a private Update() method which is needed to update the new
structures.
