compat: add window_properties object to json output of views

Some existing tools that rely on i3's json output, like the i3ipc
python module and scripts using it, break when they can't find the
window_properties object in a view.

To achieve a better drop-in replacement compatibility with i3 and
existing tools for it, we add the window_properties object to
the output of get_tree, along with the "class", "instance", and "title"
sub-fields. These three values are set to the view's class, app_id,
and title respectively.

In addition, if the view does not have a class, window_properties.class
is set to the app_id, to avoid breaking tools that expect all windows
to have a class in the X world.

Signed-off-by: Franklin "Snaipe" Mathieu <me@snai.pe>

sway/ipc-json.c

@@ -268,6 +268,28 @@ static void ipc_json_describe_view(struct sway_container *c, json_object *object
 				json_object_new_int(view_get_x11_window_id(c->view)));
 	}
 #endif
+
+	/*
+	 * The "window_properties" object is mostly redundant with other existing
+	 * fields in the view object, but is still somewhat necessary for drop-in
+	 * compatibility with existing i3 ipc tools/libs that expect them
+	 * (e.g. the i3ipc python module).
+	 */
+	json_object *window_props = json_object_new_object();
+
+	// Some tools for i3 break when windows don't have a class.
+	if (!class) {
+		class = app_id;
+	}
+
+	json_object_object_add(window_props, "class",
+			class ? json_object_new_string(class) : NULL);
+	json_object_object_add(window_props, "instance",
+			app_id ? json_object_new_string(app_id) : NULL);
+	json_object_object_add(window_props, "title",
+			c->title ? json_object_new_string(c->title) : NULL);
+
+	json_object_object_add(object, "window_properties", window_props);
 }
 
 static void ipc_json_describe_container(struct sway_container *c, json_object *object) {