Experimental: Improved selser handling for subtree-changed nodes

* So far, if a node was marked 'subtree-changed', in addition
  to descending down the tree to look for which specific child was
  modified, the serializer used regular WTS for the node itself.

  So for example, if a table cell is modified, the table itself
  is marked 'subtree-changed' and goes through normal serialization
  which leads to the table-wrapper (and all intervening nodes)
  getting normalized.

* The current patch recognizes the 'subtree-changed' diff marker
  and if the dsr values are valid, it enables serialization of the
  opening and closing tags from original source.  The separators
  are still normalized currently, but that can be a future
  enhancement.

* Since handlers are responsible for handling a node's children,
  right now, the wrapperUnmodified flag is passed into all handlers
  and the handlers decide what they want to do with the flag.

  In this patch, table/td/th/tr handlers and the html-handler
  exploit this information, and in later patches, other handlers
  could potentially be enhanced as well.

* 9 selser tests are being flagged as failures.  However, this is
  an artefact of how we test for success/failure of selser tests.
  Since we compare selser output with wt2wt output, if the wt2wt
  output itself is incorrect (because of normalization), selser
  failure/success flagging is not trustworthy. As it turns out,
  in this instance, wt2wt is at fault and the 9 selser tests are
  actually an improved output.

  This test framework limitation will be addressed separately.

* Tested on the following examples by modifying 'bar' in
  the <div> case and modifying 'a' in the table case and
  verifying that the <div> and table wikitext comes back
  without normalization.

Example 1:
<div title=foo style='color:red;'   class='foo'>bar</div>

Example 2:
{|cellspacing=0 cellpadding=3 style="color:  red"
|-
|a
|}

Change-Id: Idc7c2dbf12f0146b57edd6d053005189ac25f753
Merge "Experimental: Improved selser handling for subtree-changed nodes"