FixedPoint of sale

Point of sale: checkout, cash count and inventory fixes
We fixed VAT with discounts, cash counts with online orders, exports and several checkout details.
- VAT is calculated after discounts and VAT lines come out right.
- Online and cancelled orders count correctly in the cash count, down to the cent.
- CSV exports and failed online payments are handled correctly.
- Opening the register, checkout actions and the sale-completed notice work as expected.
- New categories are published so their products stay online.
- Number fields show 0 as a hint, without getting in the way.
- Pickup comes back after reopening a register, and an interrupted setup resumes.