[#48] PaymentVariantsWindow: Add checkbox for custom member modifiers

Elwig/Helpers/Billing/BillingData.cs

@@ -41,6 +41,10 @@ namespace Elwig.Helpers.Billing {
             get => GetConsider("consider_auto_business_shares");
             set => SetConsider(value, "consider_auto_business_shares");
         }
+        public bool ConsiderCustomModifiers {
+            get => GetConsider("consider_custom_modifiers");
+            set => SetConsider(value, "consider_custom_modifiers");
+        }
 
         public double NetWeightModifier {
             get => GetWeightModifier("net_weight_modifier", "Rebelzuschlag");