Transaction Filter "Direction"

[malfynnction]

It would be nice to add a filter parameter direction to the transaction list.
It could have the following values:

• "incoming": off-budget account -> on-budget account
• "outgoing": on-budget account -> off-budget account
• "transfer": internal account -> internal account

[morremeyer]

This is kind of related to envelope-zero/frontend#618.

We need to clearly define what we see as a transfer: Either

• a transaction with source and destination account being on-budget, or
• a transaction with source and destination account being internal

For the purpose of budgeting, the former is correct.

For the purpose of wording, I think income, spend and transfer are better choices since they line up with the wording we already use.

Thoughts?

[morremeyer]

I'll implement this as initially proposed by @malfynnction.
If we find out that it doesn't work well, we can iterate on it.