Support for "Airplane mode" keys

As we were working on audio jack notifications, and were wondering whether the type of notification we'd pop up in this case could be reused in other cases, I encountered a feature request that could now be solved easily with the rfkill D-Bus service we added to gnome-settings-daemon for the 3.10 release.

If you have keyboard buttons on your laptop to enable or disable Bluetooth, or Airplane mode, you can now use them. Note that the "UWB" toggle key will toggle the whole airplane mode mainly because no in-kernel driver uses it, and nobody remembers what UWB is.

Note that the labels and icons used are still subject to changes. In particular as you can see that the labels are too long for lower resolutions.