account_financial_report_webkit: Port to 8.0

I ported the module "account_financial_report_webkit" to 8.0.

Since report_sxw.py use the new API it's not possible anymore to add non-existing fields to a record like in "report/open_invoices.py":
```python
account.ledger_lines = ledger_lines_memoizer.get(account.id, {})
account.init_balance = init_balance_memoizer.get(account.id, {})
```
So I choice to store those values in the "localcontext" instead.