insertOrderedList: Implement merging lists beyond nested <blockquote>s

[hajimehoshi]

For exapmple,

<div ce>
  <blockquote><blockquote>f^oo</blockquote></blockquote>
  <blockquote><blockquote>ba|r</blockquote></blockquote>
</div>

must be

<div ce>
  <blockquote><blockquote>
    <ol><li>f^oo</li><li>ba|r</li></ol>
  </blockquote></blockquote>
</div>

.