Add 'Cancel' to igel-shutdown-dialog#471

The ‘igel-shutdown-dialog’ has butons to reboot or shutdown (or hibernate, if configured) but none to cancel.

Using this dialog as logoff command leads - as usually intended - to a shutdown after some seconds. Sometimes only a logoff and relogon is necessary when another collegue wants to use the IGEL. Then a ‘cancel’ would be nice - cancelling by pressing Esc ist possible but not intuitive.