Subproduct attribute with additional images for options.

Rather specific, the additional images are meant to be used when the same products may be displayed directly in the catalog and used as subproducts in a "set composer".
Translation and admin registration for the subproduct + image attribute.
Fixed a typo and unneeded, conflicting inheritance.
Made the new attribute visible in selection boxes in product and list attribute forms.
Translated strings for the new attribute.