This D-Bus property was never been added here, which caused inconsistencies
under some scenarios (e.g. coming back from suspend) if some devices were
previously paired, since _sync() would then make the bluetooth menu visible
unconditially, because of the proxied property evaluating to 'false'.
Adding this to the D-Bus interface makes sure that it's no longer undefined
and returns the right value, fixing the bug.
https://bugzilla.gnome.org/show_bug.cgi?id=789110
<interface name="org.gnome.SettingsDaemon.Rfkill"> \
<property name="BluetoothAirplaneMode" type="b" access="readwrite" /> \
<property name="BluetoothHasAirplaneMode" type="b" access="read" /> \
+<property name="BluetoothHardwareAirplaneMode" type="b" access="readwrite" /> \
</interface> \
</node>';