[NEXT] Improve PriceBag / Totals-Handling

[SamBrishes]

PriceBag is the new Totals-Calculator system, first introduced in v3.2.0 and gradually rolled out over the subsequent releases.

The following changes are intended for version 3.3.0:

• Use custom MoneyValue instead of Money to provide consistent access to values.
• Get rid of the applyDiscounts() method (find better "on-the-fly" solution)
• Increase performance, probably using some kind of caching / freezing
• Replace existing TotalsCalculator / FilteredTaxes handling and classes
• PaymentMethod discounts (ex.: -1% for cash/invoice-payments)
• New priority-based discount system with dependent values
  • New Discount System #375
  • Question to improve discount system #852
  • Discount application ordering and priority #888