Nim application10 example for GTK 3.20 is available

[Stefan Salewski <mail ssalewski de>]

I have added the Nim version of https://developer.gnome.org/gtk3/stable
/ch01s04.html#id-1.2.3.12.5 as one more example of the Nim bindings for
GTK 3.20. See

http://forum.nim-lang.org/t/2198

https://github.com/ngtk3

Tested only on 64 bit Linux yet (Gentoo AMD64, gcc 5.3). There is a
chance that it may not work with 32 bit currently, because converting
the glib and gobject C macros to Nim templates and macros has to be
done really carefully.