UX: improve import product binding

[hparfr]

In order to bind existing product "en masse" to a backend and set some "new fields" (SEO title, descriptions, images...) there is no clear way to do it.

Suggestion 1) :
Pattern Import Export can be a good help, but it's quite difficult to set it up. We can create a demo module with already defined patterns:

• pattern_simple: import products with long desc, short desc, one image, one lang, one backend
• pattern_complex: import products with multiple images, two languages, two backends