This commit ports gnome-keyring-daemon from libdbus1 to GDBus.
Unfortunately it's not really possible to split this commit into
multiple while keeping the code building.

Noteworthy is the different approach that GDBus allows us to use;
instead of filtering every message and writing our own internal message
routing, we can more naturally use generated skeletons and
register/unregister them on the bus as needed.

https://bugzilla.gnome.org/show_bug.cgi?id=622905