logind: add .ScheduleShutdown and .CancelScheduledShutdown methods

logind: add .ScheduleShutdown and .CancelScheduledShutdown methods

Add a method called ScheduleShutdown in org.freedesktop.login1.Manager
which adds a timer to shut down the system at a later point in time.

The first argument holds the type of the schedule that is about to
happen, and must be one of 'reboot', 'halt' or 'poweroff'.

The second argument specifies the absolute time, based on
CLOCK_REALTIME in nanoseconds, at which the the operation should be
executed.

To cancel a previously scheduled shutdown, the CancelScheduledShutdown()
can be called, which returns a bool, indicating whether a scheduled
timeout was cancelled.

Also add a new property called ScheduledShutdown which returns the
equivalent to what was passed in via ScheduleShutdown, as '(st)' type.