Think about savings feature

[invidian]

Currently, there is no easy way of adding savings. Savings could be inserted as fixed expense, however, this does not really reflect what they does.

Savings should:

• reduce period balance
• transfer money from one account to another (type: current -> type: savings)
• perhaps spending shouldn't be allowed from savings accounts, only transfers to current

One option would be to add 4th type of transaction, savings, but then having category for this kind of transaction doesn't really make sense. Alternatively, we could just have money transfer as transaction between 2 accounts, as mentioned in #8.