First stab at a GTK layout engine. It's missing all sorts of stuff

First stab at a GTK layout engine. It's missing all sorts of stuff
(list boxes are particularly conspicuously absent), it has no event
handling at all, and it isn't in any way integrated into pterm - you
have to build it specially using the test stubs in gtkdlg.c. But
what there is so far seems to work plausibly well, so it's a start.
Rather than browbeat the existing GTK container/layout widgets into
doing what I wanted, I decided to implement two subclasses of
GtkContainer myself, which implement precisely the layout model
assumed by the config box specification; this has the rather cool
consequence that the box can be resized and will maintain the same
layout at all times that it would have had if initially created at
that size.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2931 cda61777-01e9-0310-a592-d414129be87e