Very preliminary hack at Qt bindings for gambit scheme.  Some notes:

* hello.scm is a basically an implementation of the first two
  tutorials from http://doc.trolltech.com/4.3/tutorial.html

* webkit.scm is a simple demo of the QWebView widget.

* You will need to run the demos with LD_LIBRARY_PATH=./sqeme because
  the shared library doesn't get installed.

* bindings.cpp can be independent of scheme implementation, with all
  the implementation specific stuff going in bindings-<impl>.scm.

* Both bindings.cpp and bindings-<impl>.scm can probably be
  autogenerated from the qt headers once we decide on a standard
  mapping.

* The signals/slots implementation sucks, and is limited to signals
  and slots defined in C++ code.  This bit needs some thought.

* I've not yet tackled memory management.  Not sure if we can use the
  scheme garbage collector to reap C++ objects - if not we'll have to
  explicitly delete them (and hopefully find a pain-free way to do so
  at the right time).

* Strings are currently passed between scheme and C++ as UTF-8.  This
  is probably suboptimal.  We should find out what Qt uses internally
  (UTF 16?) and use that.