API PageSet allows generator for non-query modules

* PageSet can now be used in any action to process titles/pageids/revids
or any generator, redirects resolution, and converttitle functionality.
* action=purge proper usage of MustBePosted()
* Add supports for all pageset capabilities - generators, redirects, converttitles to
  action=purge and action=setnotificationtimestamp
* BREAKING CHANGE: ApiPageSet constructor now has two params instead of three, with only the
  first one keeping its meaning. ApiPageSet is now derived from ApiBase.
* BREAKING CHANGE: ApiQuery::newGenerator() and executeGeneratorModule() were deleted.

Change-Id: I7a3d7b6eb015d21ec1a9b9d9c6af9d97663f3f9a