[FIX] invoice_xunnel: Documents Thumbnails

This MR needs the merge of the MR !93 to keep the tests from fail, and is a solution for the issue #59

• Change thumbnail date, from the sync date to the stamped date.
• Add Name's document and Total Amount inside the thumbnail under the title.
• Total and Emmiter name are now searchable in the documents view.
• Add help for the fields in the model "ir.attachment"
• Add translations for the help and name of the fields.

New Thumbnails