Add capability to start and shutdown computers

[brydling]

The ability to start and shutdown computers should be added to a right-click menu giving the choice to start (by Wake-On-LAN) or shutdown (by help from local_supervisor) the hosts in hosts.ini either individually or all at the same time.

The updated protocol between supervisor and local_supervisor shall be backwards-compatible if possible and supervisor shall warn the user when trying to shutdown a computer running a local_supervisor that doesn't support shutdown.