power: Emit GDBusProxy-compatible PropertiesChanged signal

This is needed for gnome-shell which is now ported to use GDBusProxy;
the proxy doesn't know about the power plugin's custom Changed signal.
Given that the old interface was added for the Shell's benefit in
http://git.gnome.org/browse/gnome-power-manager/commit/?id=a85e6cf9
remove the old interface.

https://bugzilla.gnome.org/show_bug.cgi?id=667371