Client-side 'Forms Messaging' implementation: minor changes.
* removed Interface 'IFormsMessagingInterface' - this was needed in Petra 2.x where Forms inherit from a Base Form, but that doesn't apply in OpenPetra anymore.
* Changed 'FormsMessagePartner' and FormsMessageAP'' to be Structs (and no longer Classes) as there is no need for them to be Classes as the contain only data, but no functionality.