xdg-activation: distinguish activation and urgency requests

Check if the app that requested a token has provided a valid input
serial and a focused surface. Downgrade activation request to urgency
otherwise.

This is mostly in line with what other Wayland compositors decided to
do, and offers a better security than the original logic.

include/sway/desktop/launcher.h

@@ -14,6 +14,7 @@ struct launcher_ctx {
 	struct wl_listener seat_destroy;
 
 	bool activated;
+	bool had_focused_surface;
 
 	struct sway_node *node;
 	struct wl_listener node_destroy;