Bug 23354: Add a Point Of Sale 'pay' screen

Bug 23354: Add a Point Of Sale 'pay' screen

This patch adds a new Point Of Sale module to Koha's staff client front
page. The module button leads directly to a 'Pay' page giving the staff
user the ability to record anonymous payments for items that would not
normally require a patron to be registered at the library.

Test plan:
1) Enable `UseCashRegisters` via the system preferences.
2) Ensure your user has the 'manage_cash_registers' permission.
3) Add a cash register for your current branch.
4) Add at least one 'MANUAL_INV' authorized value.
5) Navigate to the new 'POS' pay page via the main menu.
6) Add an item to the 'sale' by clicking 'add' from the right side of
   the screen.
7) Note that said item was added to the table of items this sale on the
   left.
8) At this point you should be able to 'click to edit' the quantity or
   price of the item in the table on the left.
9) Enter an amount greater than the price of the item into the 'amount
   collected from patron' box.
10) Click 'Confirm'
11) Varify that the same change to give modal from the paycollect pages
    appears here.
12) Click 'Confirm'
13) Payment will have been recorded (check the database) and you will be
    back at a fresh 'Pay' page ready for the next transaction.
14) Signoff

Sponsored-by: PTFS Europe
Sponsored-by: Cheshire Libraries Shared Services
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>