Add a setting for buy vs. sell

[andreashuber69]

In a subaccount we can trade continuously, without interference from manual trades, so supporting them is a must. However, it turns out that there is no way to easily distinguish a deposit form a withdrawal in a subaccount. Both are typed as subaccount transfers, and the amount is always positive.

In a subaccount it is therefore not possible to find the intended nature of the trade (buy or sell) as we can on the main account.