Bug 24478: Use `EnablePointOfSale` preference

Bug 24478: Use `EnablePointOfSale` preference

This patch adds the `EnablePointOfSale` preference to the accounting tab
on the preferences editor and ties the display of the Point of Sale
module to it.

Test plan:
1) Enable both `UseCashRegisters` and `EnablePointOfSale` preferences.
2) Ensure you are a superuser or your user has the
   takepayment permission.
3) The Point of Sale module should be present on your staff home page
   with the above configuration, otherwhise it should not.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>