Allow multiple categories per budget

[wandertaker]

Hi,

I'd like to extend the budget feature and make it possible to create budgets with multiple categories, including system categories (#44). This would make budgets much more flexible and would allow users to use budgets for more use cases.

In a first step I'd like to implement it in a way that allows either multiple income categories or multiple outcome categories, but not both of the in the same budget (could be implemented in a follow up issue).

The change would be non breaking since it is still possible to define budgets just for single categories.

What do you think about that?

[AndreAle94]

Hello, sure! the project is open to new features and this can be very useful.

[lavinger]

this

[arneschlag]

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