Propagate leftover/overdrawn budget of one period into next

This adds a feature to reporting which propagates the budget difference of e.g. one month into the next.

Having an expense budget with 120 € each month and the following expenses (Jan 20 €, Feb 230 €, Mar 10 €) with the current report implementation one does not see how the overall budget is doing at the beginning of April as it only shows 120 €.

When enabling the new option, the picture becomes clearer: