The D-Bus specification
(https://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-properties)
defines three arguments for
org.freedesktop.DBus.Properties.PropertiesChanged.  Currently, in
dbus.el, only two are provided.

The attached patch adds the interface parameter to the test
‘dbus-test06-register-property-emits-signal’
and to the sources of the signal: ‘dbus-register-property’ and
‘dbus-property-handler’.

Thanks,
Hugh