Processing rules: Add "group with other transaction" action

[Bone008]

Use case: Automatically detecting transfers, maybe even fixed loans.

Probably useful params:

• max date difference
• other filter string (to find the matching transaction)
  • probably need to also trigger filter if a TX matching the other filter is added to be order-independent
  • not sure how to best explain that to the user
• ask for confirmation?

Maybe future group options like nominal date (#65) should be configurable, but they are probably better to implement as part of the "Set field" action instead.

[Bone008]

This could be used to auto-group transfers based on IBAN. I could also add this as a built-in feature, though it might be too restrictive.