Nim GTK3 toy chess is now available

[Stefan Salewski <mail ssalewski de>]

It is one more example which shows how GTK3 can be used with Nim:

https://github.com/StefanSalewski/nim-chess

It is currently not fully compatible with the one year old GTK3 3.15.xx
bindings from

https://github.com/StefanSalewski/

but that should be not a big problem.

As we have currently no GarbageCollector support for GTK3 bindings,
there is "pango.free(desc)" and "gobject_unref(layout)" used in
board.nim.