Added Parser::preprocess() entry point and OT_PREPROCESS output type, to expand templ...

Added Parser::preprocess() entry point and OT_PREPROCESS output type, to expand templates and other similar entities producing valid wikitext as output. The immediate application is a special page extension, shortly to be committed, but it may be used in the future to produce an intermediate cache, to improve the rendering speed of stable versions or other kinds of old revisions. Added some shortcut aliases for output type comparsions.