Fix EZP-21878: layout attribute lost when importing class package

JiRA: https://jira.ez.no/browse/EZP-21878

eZFflow's ezpage datatype does not provide  a ```serializeContentClassAttribute()``` method, which causes it to be marked as 'unsupported' when exported in a class via the package system.

This PR implements the necessary methods for the attribute to be imported/exported.