Budgets: affect budgets to new category when deleting a category

This solves the 4th point ("If a category is replaced by another which has no current budget, move the budget of the current category to the new one") of #798 (closed).

This does not affect the existing budgets to the replacement category if there are no operations for that category. Should it (it would kind of revert the previous UI modifications which removes the replacement selector when there are no ops for a category). What do you think?