Updates data type service for the way it saves pre-values so we can maintain ids if necessary - updates the pre-value editors to except a different dictionary which includes an id if necessary. This all fixes: U4-2751 Check that when publishing keys for drop down list prop eds that if we change a value that the id remains constistent

src/Umbraco.Core/Models/PreValue.cs

@@ -9,7 +9,11 @@
         {
             Value = value;
             Id = id;
+        }
 
+        public PreValue(string value)
+        {
+            Value = value;
         }
 
         /// <summary>