mediawiki.Title: Removing public setter functions. Title object should be immutable.

mediawiki.Title: Removing public setter functions. Title object should be immutable.
- To avoid repetition, instead of substituting, changed them to private helper functions
- Updated test suite to create new objects for different titles, rather than modifying the existing one.
- Adding missing unit tests, now 100% complete according to QUnit CompletnessTest
  + Test for Title.prototype.getNamespaceId
  + Test forTitle.prototype.getPrefixedText
  + Test forTitle.prototype.getExtension

Follows-up r90331 CR