Update TUI window title when changed

I wrote a TUI window in Python, and I noticed that setting its title
did not result in a refresh, so the new title did not appear.  This
patch corrects this problem.

gdb/python/py-tui.c

@@ -531,7 +531,7 @@ gdbpy_tui_set_title (PyObject *self, PyObject *newvalue, void *closure)
   if (value == nullptr)
     return -1;
 
-  win->window->title = value.get ();
+  win->window->set_title (value.get ());
   return 0;
 }