Improve flexibility of duplicating a product with/without images.

[benhuson]

The wpsc_duplicate_product_attachment filter (#1671) relates to the attachment files attached to the product (post). It currently doesn't provide control over product image IDs stored in meta such as the featured image and '_wpsc_product_gallery'.

This could be address via new wpsc_duplicate_product_image and wpsc_duplicate_product_featured_image filters.

Currently, if you duplicate a product with a featured image but use the wpsc_duplicate_product_attachment filter to opt not to duplicate the attachment files, the featured image meta will still be copied and will reference the featured image ID of the product being duplicated. The '_wpsc_product_gallery' meta is also still copied.

Possibly future enhancements also include prompting a user whether they want to duplicate images when they click the duplicate product link.

See related: #1661

[JustinSainton]

Punting to 4.1, unless you think you'll have time for this, @benhuson?

[benhuson]

When #1929 is merged this ticket can be closed.

I've set up a new issue for tackling _wpsc_product_gallery.

[JustinSainton]

Awesome, thanks @benhuson!