UI break request: logout icon

[Luca Ferretti <elle uca libero it>]

Previous icon: "gnome-logout", with the appearance of an open door with
a red arrow on it.

New icon: "system-log-out" (as per Icon Naming Spec), see
http://people.freedesktop.org/~jimmac/icons/ (note that the door with
red arrow is now used for "application-exit")

Issues:
        1. new and previous icons are currently both shipped in
        gnome-icon-theme (see bug #473082)
        2. panel should use the new icon (see bug #472254)
        
Comments?