+ * Requests a username and password through the user interface.
+ * @param obj the VLC object requesting credential informations
+ * @param username a pointer to the specified username [OUT]
+ * @param password a pointer to the specified password [OUT]
+ * @param title title for the dialog
+ * @param text text for the dialog
+ * @return Nothing. If a user name resp. a password was specified,
+ * it will be returned as a heap-allocated character array
+ * into the username resp password pointer. Those must be freed with free().
+ * Otherwise *username resp *password will be NULL.