feat(accounts): separate exchange gain and loss accounts

[barredterra]

In Company, we can specify one "Exchange Gain/Loss Account".
However, in Germany, there are two different accounts, one for gain and one for loss:

• Erträge aus der Währungsumrechnung (gain)
• Aufwendungen aus der Währungsumrechnung (loss)

Therefore, we should split "Exchange Gain/Loss Account" into two fields (easy) and use them respectively (not so easy).