Bug 11373: Show the difference between the amount collected and the amount paid of...

Bug 11373: Show the difference between the amount collected and the amount paid of a fine payment.

This patch adds a feature in the fine payment section. It allows to see the change due to patrons when the amount collected is higher than the amount paid.

Test plan :
- Apply patch.
- Select a patron with a fine.
- Go to Fines > Pay fines.
- Click the button pay.
- Choose the amount paid equal to the outstanding amount (exemple : 3$).
- Choose the amount collected to be more than the outstanding amount (exemple : 5$).
- Confirm that the change is correct (example : 5$ - 3$ = 2$).
- Click the button confirm.
- Click on Yes in the dialog box.
- Confirm that the payment has been made (example : last amount = 3$).

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>