[FIX] account: fix account.tax order

Suppose some taxes with the same sequence:

The tax are not computed in the same order in POS and SO/INV.
This is wrong in case of multiple taxes affecting the base.

-opw: 779498

addons/account/models/account.py

@@ -506,7 +506,7 @@ class AccountTaxGroup(models.Model):
 class AccountTax(models.Model):
     _name = 'account.tax'
     _description = 'Tax'
-    _order = 'sequence'
+    _order = 'sequence,id'
 
     @api.model
     def _default_tax_group(self):