Add overlay icons example to gtk-demo and pygtk-demo

["Genghis Khan" <genghiskhan gmx ca>]

Hallo,

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).

See issue at:
http://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=3066

Regards,
 --GK

-- 
()  ascii ribbon campaign - against html e-mail
/\  www.asciiribbon.org   - against proprietary attachments