Transform action component into helper component

The bureaucracy Plugin is refactored such that it uses helper plugins for its fields and actions.
This pull request transforms the pagemod action into a helper as well.

This code is only compatible with the version of code as found in:
https://github.com/splitbrain/dokuwiki-plugin-bureaucracy/pull/110
So to test you need that branch or wait that PR 110 is merged.

The new mechanism is incompatibable with the old version of the bureaucracy. But when updating via the new Extension manager of DokuWiki, the upgrade should be simple.