Clean up container title functions

* Add and use lenient_strcat and lenient_strncat functions
* Rename `concatenate_child_titles` function as that's no longer what it
does
* Rename `container_notify_child_title_changed` because we only need to
notify that the tree structure has changed, not titles
* Don't notify parents when a child changes its title
* Update ancestor titles when changing a container's layout
	* Eg. create nested tabs and change the inner container to stacking
* No need to store tree presentation in both container->name and
formatted_title

common/stringop.c

@@ -55,6 +55,20 @@ void strip_quotes(char *str) {
 	*end = '\0';
 }
 
+char *lenient_strcat(char *dest, const char *src) {
+	if (dest && src) {
+		return strcat(dest, src);
+	}
+	return dest;
+}
+
+char *lenient_strncat(char *dest, const char *src, size_t len) {
+	if (dest && src) {
+		return strncat(dest, src, len);
+	}
+	return dest;
+}
+
 // strcmp that also handles null pointers.
 int lenient_strcmp(char *a, char *b) {
 	if (a == b) {