gtk 2 updates

[Kevin Ryde]

As threatened, I've made some slightly biggish changes to get
guile-gtk using gtk 2 in the head.  It's working quite nicely (enough
for my chart program at least :-), so if you're brave then give it a
go.

I struck a few things:

- Not sure what the --with-libglade-modules configure option should do
  now, nor whether the checks on glade_gnome_init() etc are needed.
  Is it all dynamic these days?  Is there a glade expert in the house?

- It looks like the "missing" GtkType business might be able to
  disappear.  The ones that were there are ok.  There's still a few
  bits without types, eg. GTimer, but I suspect that doesn't matter.

- What does a g-wrap throw of a GError come out like?  I see
  g_quark_to_string gives say "g-file-error-quark", I wondered if the
  "-quark" is worth stripping before passing to `scm-error' or
  whatever.

- I put pango bits in with the gdk module.  Maybe it should be its own
  module, though 9 times out of 10 you only need it with gdk drawing.