Add overlay icons example to gtk-demo and pygtk-demo
In order to avoid from using specific icon set(s), which consequently
may cause to inconsistencies in Look & Feel, I suggest to add an
example code to gtk-demo, gtk3-demo and pygtk-demo of overlay icons.
Some software that have overlay icons are:
Caja, Nautilus and Thunar file manager (GTK+, folder emblem).
Blueman bluetooth manager (PyGTK, system tray).