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.

gom is now usable from JavaScript/gjs

Prodded by me while I snoozed on his sofa and with his cat warming me up, a day before the Content Applications hackfest, Florian Müllner started working on fixing a long-standing gjs bug that made it impossible to use gom in GNOME/JavaScript applications. The result of that initial research came a few days later, and is now part of the latest gjs release.

This also fixes using GtkBuilder and json-glib when the libraries create new objects for the benefit of the JavaScript code.

We can finally use gom to store user data in applications like Bolso. Thanks Florian!