window-title: implement window-title command line argument

When -t or --window-title is supplied, its required argument is treated as a
format string where `%o` is replaced by the `wlr-output`'s `name` when we set
the window title.

This uses glib to split and join the format string because our own
`string-helpers` library only has `str_join`. Otherwise using `string-helpers`
would have been preferred.

include/labwc.h

@@ -315,6 +315,8 @@ struct server {
 	struct sfdo *sfdo;
 
 	pid_t primary_client_pid;
+
+	char *window_title_fmt;
 };
 
 /* defined in main.c */