[FIX] point_of_sale: the category list now uses less space when the categories don't have pictures, freeing some screen space on tablets
[FIX] model: prevent exporting column ID of non ordinary tables

This makes no sense and can generate errors if you try to purge
ir.model.data of not existing records.
[IMP] doc/howtos/backend: finalize section on translations, and other small fixes
[IMP] doc/howtos/backend: fix calendar example