Introduce PageDao interface. Move file scanning logic to FilePageDao.

It is desirable to be able to override get_pages(...) method by plugin.
For that purpose PageDao interface is introduced.
If this interface is implemented by some of the plugin, then plugin implementation will be used.
Last loaded plugin wins.
Default implementation scanning filesystem was moved to FilePageDao class
Actual commit: 0dbe09bf8fb7ce5bf915c36730be2760393ed94c