Display class in tab title if app_id is null

This is a feature that can't work the same as i3, because there is no
real window classes in wayland. This way, we avoid null titles as much
as possible.

sway/border.c

@@ -228,15 +228,13 @@ static char *generate_container_title(swayc_t *container) {
 	for (i = 0; i < container->children->length; ++i) {
 		prev_name = name;
 		swayc_t* child = container->children->items[i];
-		const char *title = child->app_id;
-		if (child->type == C_CONTAINER) {
+		const char *title = NULL;
+		if (child->type == C_VIEW) {
+			title = child->app_id ? child->app_id : (child->class ? child->class : "(null)");
+		} else { //child->type == C_CONTAINER
 			title = generate_container_title(child);
 		}
 
-		if (!title) {
-			title = "(null)";
-		}
-
 		len = strlen(name) + strlen(title) + 1;
 		if (i < container->children->length-1) {
 			len++;