Qml app: fixed unread messages count displaying

Roles in models are not expected to change type, so they must be always
  defined and have the same type

retroshare-qml-app/src/ContactSort.js

@@ -58,6 +58,7 @@ function mergeContactsUnread()
 		var el = jsonData[i]
 		if(unreadMessages.hasOwnProperty(el.gxs_id))
 			el['unread_count'] = unreadMessages[el.gxs_id]
+		else el['unread_count'] = "0" // This must be string
 	}
 }