Support the standard org.freedesktop.DBus.ObjectManager interface.

Now that the D-Bus standard has an interface to keep track of new and
removed objects, use that instead of our own methods.  This deprecates
our D-Bus methods "GetAllClients" and "GetAllClientsWithProperties"
and the signals "ClientAdded" and "ClientRemoved", all on the server
interface "se.recompile.Mandos".

* DBUS-API: Removed references to deprecated methods and signals;
  insert reference to the org.freedesktop.DBus.ObjectManager
  interface.
* mandos (DBusObjectWithProperties._get_all_interface_names): New.
  (dbus.OBJECT_MANAGER_IFACE): If not present, monkey patch.
  (DBusObjectWithObjectManager): New.
  (main/MandosDBusService): Inherit from DBusObjectWithObjectManager.
  (main/MandosDBusService.ClientRemoved): Annotate as deprecated.
  (main/MandosDBusService.GetAllClients): - '' -
  (main/MandosDBusService.GetAllClientsWithProperties): Annotate as
                                                        deprecated.
                                                        Also only
                                                        return
                                                        properties on
                                                        client
                                                        interface.
  (main/MandosDBusService.RemoveClient): Call client_removed_signal
                                         instead of ClientRemoved.
  (main/MandosDBusService.GetManagedObjects): New.
  (main/MandosDBusService.client_added_signal): New.
  (main/MandosDBusService.client_removed_signal): - '' -
  (main/cleanup): Call "client_removed_signal" instead of sending
                  "ClientRemoved" signal directly.
  (main): Call "client_added_signal" instead of sending "ClientAdded"
          signal directly.
* mandos-ctl: Use GetManagedObjects instead of
              GetAllClientsWithProperties.  Also, show better error
              message in case of failure to connect to the D-Bus

* mandos-monitor (MandosClientPropertyCache.properties_changed):
  Bug fix; only update properties on client interface.
  (UserInterface.find_and_remove_client): Change to accept arguments
                                          from InterfacesRemoved
                                          signal.  Also, bug fix:
                                          working error message when
                                          removing unknown client.
  (UserInterface.add_new_client): Change to accept arguments from
                                  InterfacesRemoved signal.  Pass
                                  properties to MandosClientWidget
                                  constructor.
  (UserInterface.run): Connect find_and_remove_client method to
                       InterfacesRemoved signal and the add_new_client
                       method to the InterfacesAdded signal.