[6.1, account] Tax report in random order because of missing tax code sequences

[Stefan Rijnhart (Opener)]

As of 6.1, tax codes in the tax report (on the same level in the hierarchy) are now sorted by 'sequence'. Previously, the tax report was sorted by account_tax_code.code. The sequence in new installations is derived from tax code templates, so existing tax codes cannot be updated by XML ID. Therefore, assign a sequence based on the tax code's code.