0024023: Revamp the OCCT Handle -- plugin

0024023: Revamp the OCCT Handle -- plugin

Definition of PLUGINFACTORY function changed to return Standard_Transient* instead of Handle(Standard_Transient), which cannot be returned by C-style function.
Default implementation of PLUGINFACTORY() instantiated by macro PLUGIN() is corrected accordingly.
Methods Factory() in persistence packages are made returning const & to handle; this is to reflect the fact that returned handle should point to existing object (usually statically allocated) rather than new one.