Remove inheritence in favour of composition

Breaking Changes

• Bindings now use System.ComponentModel INotifyPropertyChanged and INotifyCollectionChanged
• Removed ObservableList. Use ObservableCollection instead
• Scene items no longer inherit from ObservableNode
• Scene items still have to implement IObservableObject
• Scene items must create an instance of BindingContext and register bindings against the context
• BindingContext is now the context for validations. Use this for HasError and for subscribing to the ControlValidationChanged event
• New base classes ControlViewModel and NodeViewModel can be used isntead of implementing IObservableObject
• Bindings require a node reference instead of a path

New Features

• Code generators such as PropertyChanged.SourceGenerator can be used to implement INotifyPropertyChanged
• List bindings now support Move, Replace, and Insert events
• Added support for custom control bindings. Custom controls must implement IObservableObject
• Custom controls can be bound by targeting a property that raises the OnPropertyChanged event
• Nodes can be bound using extensions methods for BindProperty etc.