Use common marshalling routines instead of having widget-private ones. It

Use common marshalling routines instead of having widget-private ones. It

Use common marshalling routines instead of having widget-private ones.
It compiles & links.

testgtk segfaults. I'm convinced that is Somebody Else's Problem
(marshalling routine gets func_data=0x0 and blithely passes it on) but the
fact that it happens now & not before makes me wrong :-)