Windows: make the wait dialog mechanism more generic in order to reuse it more widely across VeraCrypt.
Windows: implement wait dialog for the change password operation.
Windows: use the correct window handle for creating message boxes. This became important after the introduction of the wait dialog in order to avoid having message boxes behind the wait dialog.
Windows: implement wait dialog for the backup/restore header operations.
Windows: use the correct parent window when changing the password of the encrypted system partition.