DOM-Diff: Add additional diff markers for text nodes.

* This pass adds diff-markers before text and comment nodes
  that have been modified in some way (inserted or changed).

* WTS or selser itself has not been changed in this patch.

  A series of prior patches have fixed selser and WTS to better
  handle additional mw:DiffMarker metas.

* There have been some changes to selser tests results as a
  result of changes to the dom-diff algorithm. 30 newly "failing"
  tests and 10 newly "passing" tests.

  - All selser test result changes that have a "[[4,.." in
    the change tree are for tables where a comment has been
    added as the first child of the table. The new selser output
    in all these scenarios is actually an improved version compared
    to the old selser output (and the WTS output).

    "{|<!--comment-->" INSTEAD OF "{|\n<!--comment-->"

  - As the following output shows, 26 of the additional "failing"
    tests are actually a version of this change.

[subbu@earth tests] git diff | grep "^+add" | wc
     30     549    7008
[subbu@earth tests] git diff | grep "^+add" | grep "\[\[4" | grep "{|" | wc
     26     458    5309
[subbu@earth tests] git diff | grep "^+add" | grep "\[\[4" | grep -v "{|" | wc
      0       0       0

  - As for the remaining 4, three of the tests are changes to
    italic/bold tests (and complex edits). The new selser output
    seems better and less dirty than the older version.

  - Some of the newly "passing" tests (10) can be considered a
    degradation, but the rest of them are newline changes and just like
    the table comment changes above, the new output can be considered
    an improvement and more readable (with newlines before paragraphs).

  - Overall, even without any changes to the WTS / selser algorithm, the
    changes to dom-diff seems to improve the quality of selser output.

Change-Id: I332595e3763e9c8b29f7da552ff328a9c85bad12