New Drawing/Events/XInput tutorial

[Owen Taylor <owt1 cornell edu>]

I've put a new tutorial chunk at:

  http://www.msc.cornell.edu/~otaylor/gtk-gimp/tutorial/

It goes through creating a simple drawing program, covering
event handling (not too completely), the basics of drawing
and DrawingArea widgets (less completely), and using
XInput (sort of completely). 

Although one example is used throughout, it might be worth
splitting the XInput stuff off from the rest when integrating
it into the tutorial, since the rest of it is pretty basic
stuff, while XInput is a bit more specialized topic. (On
the other hand, once people see how easy supporting XInput
is, maybe everybody will want to xinputize their programs...)

I've also added the stuff about widgets that don't have
windows into this portion, since it seems to fit more
naturally there than in it's original location.

Regards,
                                        Owen