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

include/stringop.h

@@ -1,5 +1,6 @@
 #ifndef _SWAY_STRINGOP_H
 #define _SWAY_STRINGOP_H
+#include <stdlib.h>
 #include "list.h"
 
 #if !HAVE_DECL_SETENV
@@ -14,6 +15,10 @@ char *strip_whitespace(char *str);
 char *strip_comments(char *str);
 void strip_quotes(char *str);
 
+// strcat that does nothing if dest or src is NULL
+char *lenient_strcat(char *dest, const char *src);
+char *lenient_strncat(char *dest, const char *src, size_t len);
+
 // strcmp that also handles null pointers.
 int lenient_strcmp(char *a, char *b);