Add ICNS (Apple Icon Image) plugin.

Change-Id: I98f79d781e5986ee5602438e02d761c7f5a77217
Reviewed-by: Ivan Komissarov <ABBAPOH@me.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>