Updated icon for the Power panel

[Cosimo Cecchi <cosimoc gnome org>]

Hi everyone,

I'm asking for permissions to push the patch attached to [1]. It changes
the icon of the Power panel to the one shipped in gnome-icon-theme,
which looks so much better.

Screenshots for comparison:
before: http://people.gnome.org/~cosimoc/cc.png
after: http://people.gnome.org/~cosimoc/cc2.png

[1] https://bugzilla.gnome.org/show_bug.cgi?id=645535

Thanks,

Cosimo