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;

types/wlr_xdg_activation_v1.c

@@ -32,6 +32,9 @@ void wlr_xdg_activation_token_v1_destroy(
 	if (token->timeout != NULL) {
 		wl_event_source_remove(token->timeout);
 	}
+
+	wlr_signal_emit_safe(&token->events.destroy, NULL);
+
 	wl_list_remove(&token->link);
 	wl_list_remove(&token->seat_destroy.link);
 	wl_list_remove(&token->surface_destroy.link);
@@ -258,6 +261,7 @@ static void activation_handle_get_activation_token(struct wl_client *client,
 	wl_list_init(&token->link);
 	wl_list_init(&token->seat_destroy.link);
 	wl_list_init(&token->surface_destroy.link);
+	wl_signal_init(&token->events.destroy);
 
 	token->activation = activation;
 
@@ -376,6 +380,7 @@ struct wlr_xdg_activation_token_v1 *wlr_xdg_activation_token_v1_create(
 	// Currently no way to set seat/surface
 	wl_list_init(&token->seat_destroy.link);
 	wl_list_init(&token->surface_destroy.link);
+	wl_signal_init(&token->events.destroy);
 
 	token->activation = activation;