follow up r60832 and follow up r60763

follow up r60832 and follow up r60763

* Don't set Parser::$mTitle to random garbage.
* Remove ParserOutput::$displayTitle, make setDisplayTitle() and
  getDisplayTitle() wrappers for their *TitleText() equivalents.
* Remove Parser::$mDo*Convert member variables, move test for
  $mDoubleUnderScores[] directives closer to the action.
* Remove bogus "global $wgContLang".
* Use accessor to get at $mConvRuleTitle
* Fix up showtitle option in parserTests.inc
* TODO: refactor FakeConverter class away