Refactor ICNS plugin to prepare it for icon variants support

Change-Id: I8bc799ade204072bdc169180154c2c2b021fd44e
Reviewed-by: Ivan Komissarov <ABBAPOH@me.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>