In previous versions, string was converted to button title and wasn't displayed
Now it was being shown, so this converts it back to the button title
TT29027
* Tests
* PEP8
* Use invoice._convert_to_write(invoice._cache). This way, the onchange will be inheritable and will add here also the added values
* better get_context_values
* unify method compute_balance
* open move lines linked to balance