Add basic support for working with a widget's GdkVisual

This is a neat little toy program IMHO. It builds off of my addition of `gdk.EventKey` and (I think) some improvements that were made to the signal/callback mechanism sometime in the recent past (at least I think--I recall having trouble getting the `draw` signal to work a few months ago). It really shows how far `gotk3` has come in that time.