Port from dbus-python to Gio GDBus API

[lmacken]

https://developer.gnome.org/gio/stable/gdbus-convenience.html
https://developer.gnome.org/gio/stable/gdbus-lowlevel.html

[ralphbean]

This might suck for the gnome haters out there. I'm 👍 for it, but the best of worlds would be if we could include a compat layer that supported both raw dbus and this gio gdbus stuff.

[lmacken]

We already use the new Gio stuff in the daemon, as well as dbus-python. So porting would allow us to drop a dep without adding any new ones.

[ralphbean]

👍