Allow an "initial balance" of accounts

[humdingerb]

As you normally don't start using CapitalBe right from opening a new bank account, we should allow for an "Initial balance".

Instead of introducing some sort of special category for it that doesn't appear in reports etc., how about making it an item of an account's "setting"? I haven't had a look at the needed code at all, just thinking about it in general...

[humdingerb]

Implemented with 385a0c206567d53ba47e653e0fa7de45a84c13dd ff.