Add common interface for preference values

main/src/com/google/refine/preference/PreferenceValue.java

@@ -0,0 +1,25 @@
+package com.google.refine.preference;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+
+/**
+ * Interface to be extended by all objects stored 
+ * in the preferences. This ensures that their full class
+ * name is serialized with them. They should implement
+ * Jackson deserialization as usual.
+ * 
+ * @author Antonin Delpeuch
+ */
+
+@JsonTypeInfo(
+        use=JsonTypeInfo.Id.CLASS,
+        include=JsonTypeInfo.As.PROPERTY,
+        property="class")
+public interface PreferenceValue {
+	
+	@JsonProperty("class")
+	public default String getClassName() {
+		return this.getClass().getName();
+	}
+}