xdg-activation-v1: add data field and emit token destroy events

The data field is useful to track metadata about a token. The destroy
events are useful for compositors that track application startup to
let them know they can stop doing that.

include/wlr/types/wlr_xdg_activation_v1.h

@@ -22,6 +22,12 @@ struct wlr_xdg_activation_token_v1 {
 	char *app_id; // can be NULL
 	struct wl_list link; // wlr_xdg_activation_v1.tokens
 
+	void *data;
+
+	struct {
+		struct wl_signal destroy;
+	} events;
+
 	// private state
 
 	char *token;