background: convert to GDBus (and fix delayed background drawing)

For some reason the signal filter with plain DBus wasn't working
correctly, leading to no background when nautilus was for whatever
reason not autolaunched, and show-desktop-icons was TRUE.  In any
case it should be GDBus not libdbus, so port it.  That also
happens to fix the signal listening issue.

https://bugzilla.gnome.org/show_bug.cgi?id=636233