Show a placeholder for encrypted messages

Tells the user that the message could not be decrypted because the key
was not shared with the device. At the moment, this is technically not
completely correct, but it will be when libQuotient supports reading
encrypted messages

src/messageeventmodel.cpp

@@ -490,6 +490,9 @@ QVariant MessageEventModel::data(const QModelIndex &idx, int role) const
         if (evt.isStateEvent()) {
             return "state";
         }
+        if (is<const EncryptedEvent>(evt)) {
+            return "encrypted";
+        }
 
         return "other";
     }