Use D-Bus properties instead of our own methods.

* mandos (Client._datetime_to_milliseconds): Renamed to
                                             "_timedelta_to_milliseconds".
                                             All callers changed.
  (dbus_service_property): New decorator for D-Bus properties.
  (DBusPropertyException, DBusPropertyAccessException,
  DBusPropertyNotFound): New D-Bus exception classes.
  (DBusObjectWithProperties): New; extends "dbus.service.Object" with
                              support for properties.
  (ClientDBus): Inherit from, and call up to, "DBusObjectWithProperties".
  (ClientDBus.CheckedOK, ClientDBus.GetAllProperties,
  ClientDBus.SetChecker, ClientDBus.SetHost, ClientDBus.SetInterval,
  ClientDBus.SetSecret, ClientDBus.SetTimeout, ClientDBus.Enable,
  ClientDBus.StartChecker, ClientDBus.Disable,
  ClientDBus.StopChecker): Removed, replaced with properties.
  (ClientDBus.IsStillValid): Removed, superfluous.
  (ClientDBus.name_dbus_property,
  ClientDBus.fingerprint_dbus_property, ClientDBus.host_dbus_property,
  ClientDBus.created_dbus_property,
  ClientDBus.last_enabled_dbus_property,
  ClientDBus.enabled_dbus_property,
  ClientDBus.last_checked_ok_dbus_property,
  ClientDBus.timeout_dbus_property, ClientDBus.interval_dbus_property,
  ClientDBus.checker_dbus_property,
  ClientDBus.checker_running_dbus_property,
  ClientDBus.object_path_dbus_property,
  ClientDBus.secret_dbus_property): New D-Bus properties.