Amount widget +/- has floating point rounding artifacts #4500

It goes from 21000000.00000000 to 20999999.99990001. The reason for this is that we use a QDoubleSpinWidget, and errors accumulate in the lower digits every time when subtracting the step.

Solution: Instead of QDoubleSpinWidget we should create a spin widget subclass that uses fixed-point amounts.

Fixed by #4556