Improve serialization for FilteringCategory

[TechnikEmpire]

Presently, FilteringCategory objects are wrapped in a CategorizedFilteredRequestsViewModel object, which are pushed to ObservableCollections and served across multiple views to be displayed in sub-views.

Since creation of FilteringCategory objects is purely a user driven event, a control exists where the user can do this. Results are given through category creation events. This creates a problem for serialization, as we don't want to serialize the view model, but just the model. The model, however, is not directly publicly visible (or shouldn't be) from the view model. But, we've been very stOOPid and done just for the sake of allowing quick and dirty serialization of program state.

Fix this.