Add a way to get the memory consumption of a string list

This is just for tests, and not meant to be merged.
I want to be able to compare the data structure size
for various string list implementations.

gtk/gtkstringlist.c

@@ -581,3 +581,9 @@ gtk_string_list_get_string (GtkStringList *self,
 
   return objects_get (&self->items, position)->string;
 }
+
+guint64
+gtk_string_list_get_size (GtkStringList *self)
+{
+  return sizeof (GtkStringList) + (self->items.end_allocation - self->items.start) * sizeof (gpointer);
+}

gtk/gtkstringlist.h

@@ -69,6 +69,9 @@ GDK_AVAILABLE_IN_ALL
 const char *    gtk_string_list_get_string      (GtkStringList         *self,
                                                  guint                  position);
 
+GDK_AVAILABLE_IN_ALL
+guint64         gtk_string_list_get_size        (GtkStringList         *self);
+
 G_END_DECLS
 
 #endif /* __GTK_STRING_LIST_H__ */