wayland: fix fontconfig monitoring
Submitted by Matthias Clasen
Under X11, we are using and X setting that contains a 'fontconfig timestamp', and that gets updated by gnome-settings-daemon. This doesn't work under Wayland, since we only monitor those xsettings that are backed by a gsetting.
After some discussion on #gtk+, we suggest to use a dbus signal instead. It can be emitted by the same gsd plugin that maintains the xsettings.