WIP: Optimize GtkListBox for sequential access

GSequence always iterates from 0 to log N when retrieving the Nth element. That means writing a for-loop like this:

  for (i = 0; i < N; i++)
    row = gtk_list_box_get_row_at_index (listbox, i);

is accidentally O(n * log n). Ooops!

Bring the GListStore optimization for sequential access.