Support idle-inhibit Wayland protocol
idle-inhibit from wayland-protocols is the widely accepted way of inhibiting idle actions (screensaver, lock, display off) on Wayland.
GtkApplication currently only uses some D-Bus interfaces (
org.xfce.Session.Manager) which leaves behind simple compositors that don't expose these interfaces.
GTK should use
idle-inhibit when none of these interfaces are present.