Feature/history in a line

I reduced dual undo stack into one with stackOffset.
This patch also fixes wrong bookmark implementation.

And changed recording undo stack timing from 'before modifying text' to 'after modifying text'. The first undo stack is recorded while initiating summernote.
혹시 요 부분 `list.initial` 뺐는데... 이유가 있었어요~?
`offsetPath`의 본래 의도는 `ancestor`로 부터 자신의 위치를 찾는거라
`listAncestor`에 `ancestor`도 포함되어 제거했었거든요.