Add concept of month closing

[TheAxelander]

The idea is to prevent all changes in previous months like adding or modifying Transactions, Buckets etc. This could prevent unforeseen Data Inconsistencies and unwanted situations. Having month closing all Balances and other figures could be saved as a snapshot which would speed up calculations.

Before closing a month the following things should be ensured:

• All Transactions assigned to a Bucket
• No negative Bucket Balance

[SystemFiles]

This would be very helpful! +1