Budgets: allow 0 threshold (fix #537)

0 is now a valid number value for budgets thresholds.

Since we can't make any progress assumption on 0 nor tell if it is a good thing to have a value over 0 (most of the time I guess you'd use 0 to check in a balance that every expense is matched with an income and vice-versa), the state will either be success (amount = 0 ) or danger (amount != 0).

When there is no threshold the amount input now just appears empty.