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.freedesktop.portal.Inhibit, org.gnome.SessionManager, 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.