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 :-)
Start of marshalling centralization.
Please check this over for sanity. I think the perl script and Makefile might
need fixing up to allow builddir != srcdir
I will start converting all the widgets to use this scheme if no problems
arise.