[FIX] barcode_interface: when loading widget, picking_type_id was sometime not correctly set
[IMP] fields: do not copy field objects anymore, but make new instances instead

Because of the parameter overriding mechanism implemented by fields, it is no
longer necessary to copy field objects.  It is even better to no copy them in
the case of related fields.
[IMP] fields: rename attribute 'copyable' into 'copy'
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64