[16.0][FIX] account_financial_report: fix the order of vat tag report lines

[dhx]

• Currently the lines of the vat tax tag report are not properly sorted leading to similar tags (only differing by their sign) ending up far apart making it difficult to see when values have to be subtracted from another to get the final result.
• This patch sorts the lines first by the code (currently always empty) second by the name of the tag (without the sign) and only third by the sign (to always have the '+' in front of the '-' tag)