sortlistmodel: Make sort stable again

Previously, the sort was not stable when items were added/removed while
sorting or the sort algorithm was changed.

Now the sort looks at the item position (via the key's location in the
keys array) to make sure each comparison stays stable with respect to
this position.

gtk/gtksortlistmodel.c

@@ -255,7 +255,11 @@ sort_func (gconstpointer a,
   gpointer *sb = (gpointer *) b;
   int result;
 
-  return gtk_sort_keys_compare (data, *sa, *sb);
+  result = gtk_sort_keys_compare (data, *sa, *sb);
+  if (result)
+    return result;
+
+  return *sa < *sb ? -1 : 1;
 }
 
 static gboolean