[1.5.0] Add support for budgets with multiple assigned categories

It should be possible to assign multiple categories to a budget (for example, if you want to create a budget for all expenses in a folder).

This includes:

• By default, "[1, 2, 3] categories" is displayed instead of the category name in the list

• Optionally, it should be possible to add a name to a budget that will be displayed instead

• In the filtered transaction list (e.g. up to 3 icons of the individual categories or a standard icon for the "group") should be displayed in the header view.