Lab/ConfigurationEditor: Complete architecture revamping. (...)

- ConfigurationEditor now has a SINGLE ViewModel, acting as controller for ALL ConfigurationManager-related items and changes.
* This avoids having to create and synchronize child VMs for things like Layers and Items.
- Clear Optionals will now clear empty layers.
- Empty layers no longer need confirmation before deletion.
- New NuGet dependency for System.Windows.Interactivity.
- Changed ViewModelBase to include CallerMemberName, since we're now on 4.0.