Add hook to allow globals to send cold-plug events to new clients.

This lets us boot strap the client side state cache.  This commit also adds the first user
of this feature, an output object that represents the current output.  Very simple
at this point, but will grow to something more like RandR 1.2.

wayland-protocol.c

@@ -83,3 +83,14 @@ WL_EXPORT const struct wl_interface wl_input_device_interface = {
 	0, NULL,
 	ARRAY_LENGTH(input_device_events), input_device_events,
 };
+
+
+static const struct wl_message output_events[] = {
+	{ "presence", "uu" },
+};
+
+WL_EXPORT const struct wl_interface wl_output_interface = {
+	"output", 1,
+	0, NULL,
+	ARRAY_LENGTH(output_events), output_events,
+};