LPS-46232 Staging controller classes refactoring

Hey Brian,

We are trying to move forward with the staging controller classes to be as minimal as necessary, and they should contain as less state as necessary, because they usually just contain controlling parts, the PortletDataContext is the tool for tracking the state of the staging.
In the lights of this, Dániel has refactored these classes (*Exporter/*Importer)

Thanks,

Máté