Back button to the checkout

[Lôc Cosnier]

Hi all,

I added a back button to the checkout.
I think this may be a good idea if the customers have made a mistake or
want to be sure what they have choosen, they can return quickly without
waiting for the end of the checkout.

It is possible to join the project and commit changes.

I try to update French translations if needed and add Dutch translation
this week-end.

Loc