Databinding improvements

Main points:

• Separation of the form model from the layout
• Keeping the transient changes directly in Vaadin fields, avoiding attempts to replicate JCR session across requests (aka JCRNodeAdapter)
• Implementation of the complex fields (composites, multi-fields etc) based on the form/editor concept
• Vaadin 8 powered implementation

public interface EditorDefinition<T> extends ViewDefinition<EditorView<T>> {
}

/**
 * Base interface of an editor.
 */
public interface EditorView<T> extends UiFrameworkView {

    void populate(T item);

    void write(T item);

    List<BinderValidationStatus<?>> validate();
}

public interface FormDefinition<T> extends EditorDefinition<T> {

    List<EditorPropertyDefinition> getProperties();

    LayoutDefinition getLayout();

    ...
}

Comments on the structure:

• The only thing base EditorDefinition enforces is that the related view is an editor view to extend interface EditorView
• The latter does not enforce any particular structure of the implementation, just basic data-binding capabilities
• FormDefinition and the related FormView are an extension of the editor concept. Form assumes the presence of the properties and the layout definition.
• The EditorPropertyDefinition is either a 'simple' FieldDefinition or a ComplexPropertyDefinition which is a some form of editor view definition plus the definition of the strategy of obtaining the child item/node the complex field to bind to.
  
  

public interface ItemProviderStrategy<T> {

    Optional<T> read();
}