dbus: Raise PropertiesChanged on org.freedesktop.DBus.Properties

dbus: Raise PropertiesChanged on org.freedesktop.DBus.Properties

wpa_supplicant dbus objects are currently difficult to use with
GDBusProxy. This is because they do not follow the dbus standard in
emitting the PropertiesChanged signal on the standard D-Bus properties
interface, so GDBusProxy stores stale property values.

Fix this by raising PropertiesChanged on the standard interface.
For backwards compatibility, the existing custom PropertiesChanged
signals are not removed; I anticipate them being removed in a future
major release.

See
http://lists.freedesktop.org/archives/dbus/2011-August/014593.html
for more info.

Signed-off-by: Daniel Drake <dsd@laptop.org>