[ADD] [7.0] 'pos_backup_draft_orders' module

Add a new module to allow users to backup (save in database) draft pos order.
So, in the Point of Sale module, add a button "backup draft Order".
This can be usefull because some features are not available in Front End Point of sale for the time being. (Invoicing for exemple, or changing pricelist).
For security reasons it can be used, if the user has to shutdown his computer or to restart his browser.