Show cached conversations list before getting list from the server

lib/models/conversation.dart

@@ -7,7 +7,7 @@ import 'package:meta/meta.dart';
 ///
 /// @author Pierre HUBERT
 
-class Conversation extends CacheModel {
+class Conversation extends CacheModel implements Comparable {
   final int ownerID;
   final int lastActive;
   final String name;
@@ -56,4 +56,9 @@ class Conversation extends CacheModel {
       ConversationTableContract.C_MEMBERS: members.join(","),
     };
   }
+
+  @override
+  int compareTo(other) {
+    return other.lastActive.compareTo(this.lastActive);
+  }
 }