Factor out mark filtering/sorting for use in _dumpMarks()
Allow edits *before* a deleted item to be relative to it
Expanded on comment to clarify why it's ok to keep the old afterID
Update README.md
Merge pull request #5157 from adobe/nj/issue-5154

[LiveHTML] Allow edits *before* a deleted item to be relative to it
Merge pull request #5150 from adobe/nj/mark-sort-cleanup

Factor out mark filtering/sorting for use in _dumpMarks()