dbus: Replace StateChanged with PropertiesChanged signal

dbus: Replace StateChanged with PropertiesChanged signal

The actual supplicant state is exposed via a property on the interface
object. So having a separate signal StateChanged for notifying about
changes is a bad idea. The standard PropertiesChanged signal should be
used for this.

The advantage of StateChanged signal was that it includes the previous
state, but not even NetworkManager is making use of this. And tracking
the old state via the property and this signal is easily possible anyway.