Implemented displaying errors.

ucalc/Models/BillingModel.cs

@@ -0,0 +1,21 @@
+using UCalc.Data;
+
+namespace UCalc.Models
+{
+    public class BillingModel : Model
+    {
+        public LandlordModel LandlordModel { get; }
+
+        public BillingModel(Billing billing)
+        {
+            LandlordModel = new LandlordModel(billing.Landlord);
+
+            Properties = LandlordModel.Properties;
+        }
+
+        public override void Apply()
+        {
+            LandlordModel.Apply();
+        }
+    }
+}