Improve the shutdown menu by adding support for Sleep modes and use of dbus interfaces

I've added a new module to deal with the process of shutting down/sleeping freevo.

This has a couple of advantages for plugins if they want to know when freevo shuts down or if they want to stop freevo shutting down the system (for example something is being downloaded etc).

Finally this means that there is no longer a need to use sudo to shutdown the system for most people as the new module will use either logind or ConsoleKit/UPower dbus interfaces to shutdown/reboot/suspend/hibernate the system.