Add support for reading icon variants in ICNS plugin

Change-Id: I68395ac4e9604d852405643710aa79585974b3e3
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>