Work on extrafields

New : can define that an extrafield is always editable (regardless of the document status)
Qual : display of extrafields on objects is now in a single tpl file (proposal, order, invoice, fichinter, contrat, supplier order, supplier invoice)
Fix : extrafields were not working on supplier invoices
Cosmetic : the icon displayed is now the same for extrafields as the standard fields (icon on the left of the value and no "Modify" work next to ir)