fix Undefined variable $id

implement PageFile class

PageFile class handles wiki text file and its change management for specific page, main part comes from `function saveWikiText()`. This ensures and enables to use one **Pag

implement PageFile class

PageFile class handles wiki text file and its change management for specific page, main part comes from `function saveWikiText()`. This ensures and enables to use one **PageChangeLog** instance during a page save process among class methods and relevant plugins.

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rename namespace Media to File

This is a bit more generic and allows us to have additional File
abstractions in here later.