Add a price setting to round percentage-based prices to the nearest number

[mgesing]

This enables merchants to harmonize their prices in relation to manually maintained prices. Scope:

• Tier prices of type TierPriceCalculationMethod.Percental.
• Depending prices of type DependingPriceCalculationMethod.Percental.
• Attribute price adjustments where TierPriceCalculationMethod.Percental is to be considered.
• Entities with UsePercentage == true like discounts, shipping rate by total and weight.