[1.5.0] Add automatic credit card billing

For a traditional credit card (where, for example, the balance is paid at the end of the month from a bank account), the user currently has to do the following steps:

• Create two accounts (one for the bank account and one for the credit card).

• When creating new transactions, he has to select the credit card in order to have the amounts debited directly from the card.

• When the payment date arrives, he will have to create a transfer by entering the amount to be debited and selecting the regular bank account as the "From Account" and your credit card as the "To Account".

• The amount to be debited must be calculated manually (or through the application by filtering the transactions). However, it is not very user friendly to do this every billing cycle.

Since this is currently a manual process, it might be useful to be able to add the billing period and the associated bank account to the credit card so that the billing is done automatically by the app e.g. like this:

• If the user edits an account of type "Credit Card", he will see a new "Billing" section, where he can define the date on which the amount of the current billing period will be debited, as well as the account from which the amount will be debited.

• When the target date is reached, the application automatically creates a new transaction (which is not included in the statistics so as not to distort them) to debit the amount from the specified account so that the credit card is balanced.

• To avoid duplicate transactions, the account internally stores a flag to maintain the last billing date.