Make KeyIndexedObject serializable, in order to allow correct seriali…

…zation of its subclass UserProfile

src/org/openstreetmap/josm/plugins/mapillary/model/KeyIndexedObject.java

@@ -1,10 +1,13 @@
 // License: GPL. For details, see LICENSE file.
 package org.openstreetmap.josm.plugins.mapillary.model;
 
+import java.io.Serializable;
+
 /**
  * An object that is identified amongst objects of the same class by a {@link String} key.
  */
-public class KeyIndexedObject {
+public class KeyIndexedObject implements Serializable {
+  private static final long serialVersionUID = 7762862623109416405L;
   private final String key;
 
   protected KeyIndexedObject(final String key) {

src/org/openstreetmap/josm/plugins/mapillary/model/UserProfile.java

@@ -1,14 +1,12 @@
 // License: GPL. For details, see LICENSE file.
 package org.openstreetmap.josm.plugins.mapillary.model;
 
-import java.io.Serializable;
-
 import javax.swing.ImageIcon;
 
-public class UserProfile extends KeyIndexedObject implements Serializable {
+public class UserProfile extends KeyIndexedObject {
   private static final long serialVersionUID = 4695718596660412790L;
-  private String username;
-  private ImageIcon avatar;
+  private final String username;
+  private final ImageIcon avatar;
 
   public UserProfile(String key, String username, ImageIcon avatar) {
     super(key);