Migration Manager extension support

This ticket is related to this propsal discussion on xwiki-devs.

The idea is to merge the concepts of the Extension Manager and Migration Manager. Extensions should be able to provide individual data migrations to add the possibility of altering data relevant to this extension. These should be executed after installation or upgrade of an extension.

Currently, the Migration Manager (still residing in the oldcore) does not support external migrations and only a version number for XWiki migrations is stored in the database. It has to be refactored to it's own module and requires the functionality to store multiple version numbers with specific identifiers in the database for versioning migrations for individual extensions.