Add simple GtkDrawingArea example

[sonnyp]

Add an example for GtkDrawingArea, with a very simple cairo example. The example should primarily demonstrate the GTK widget, not cairo.

https://docs.gtk.org/gtk4/class.DrawingArea.html

[sonnyp]

@andyholmes wdyt of #296 ?

[andyholmes]

Ah yes, that's good. The cairo example might be even a little too complex, but that's definitely enough.

Cairo is a whole topic with vectors and so on, but as long as the demo shows how to get started (including special cases like cr.$dispose() in GJS) that's good.