(Bug 57854) Handle linktrail/prefix nowiki escaping in selser mode

* The bug report (test case reproduced below) amply demonstrates
  the problem.

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[subbu@earth tests] echo "[[foo]]" > /tmp/wt
[subbu@earth tests] node parse < /tmp/wt > /tmp/old.html
[subbu@earth tests] sed 's/<\/a>/<\/a>ls/' < /tmp/old.html > /tmp/new.html
[subbu@earth tests] node parse --selser --oldtextfile /tmp/wt --oldhtmlfile /tmp/old.html < /tmp/new.html
[[foo]]ls
[subbu@earth tests] node parse --html2wt < /tmp/new.html
[[foo]]<nowiki/>ls
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* Linktrail/prefix nowiki escaping was tied to the link-handler.
  But the link-handler doesn't trigger in selser mode which means
  in selser mode, nowikis were not being added where necessary.

* This patch associates this nowiki escaping with text nodes which
  means independent of how the associated links were serialized,
  nowikis are added where necessary.

* 156 selser tests are now passing!

Change-Id: I120ac3e124756531d8a8f42456f2e7e7587367c3
Merge "(Bug 57854) Handle linktrail/prefix nowiki escaping in selser mode"