ipc: add pid information for views in layout tree

include/sway/tree/view.h

@@ -61,6 +61,8 @@ struct sway_view {
 	struct sway_container *container; // NULL if unmapped and transactions finished
 	struct wlr_surface *surface; // NULL for unmapped views
 
+	pid_t pid;
+
 	// Geometry of the view itself (excludes borders) in layout coordinates
 	double x, y;
 	int width, height;

sway/ipc-json.c

@@ -221,6 +221,8 @@ static const char *describe_container_border(enum sway_container_border border)
 }
 
 static void ipc_json_describe_view(struct sway_container *c, json_object *object) {
+	json_object_object_add(object, "pid", json_object_new_int(c->view->pid));
+
 	const char *app_id = view_get_app_id(c->view);
 	json_object_object_add(object, "app_id",
 			app_id ? json_object_new_string(app_id) : NULL);

sway/tree/view.c

@@ -470,6 +470,7 @@ static struct sway_workspace *select_workspace(struct sway_view *view) {
 		wl_resource_get_client(view->surface->resource);
 	wl_client_get_credentials(client, &pid, NULL, NULL);
 #endif
+	view->pid = pid;
 	ws = root_workspace_for_pid(pid);
 	if (ws) {
 		return ws;