Components API module and Converter improvements

New features

• Created a new module - flow-components, that contains the component API for the Vaadin webcomponents. Those components are built on top of the generated API, and provide all the features and documentation that are not possible to auto generate. The first released component is VaadinButton.
• ModelConverters can now work with list of beans.
• ModelConverters now have a default implementation for the getApplicationType and getModelType methods, so the developer doesn't need to provide those in most cases.

Fixes

• QueryParameters now work with parameters without values (like in http://somedomain?parameter).
• Fixed an error with NPM builds that prevented clean projects to run.
• Fixed a problem that prevented two-way binding to work for beans that were set from the server-side.
• Update the used versions of NPM, Yarn and Node across the modules.

Breaking changes

• With the addition of the flow-components module, the components from the flow-generated-components were all renamed to have a Generated prefix. So, for example, the generated VaadinButton is now called GeneratedVaadinButton, and the final implementation is called just VaadinButton. Also, the base package of the generated components were changed to com.vaadin.generated.

Demos

• Added a demo for ModelConverters.

Tutorials

• Added a tutorial for ModelConverters.
• Updated the tutorial about Using Beans with a PolymerTemplate Model clarifying the usage of interfaces as models.
• Updated the tutotrial about Handling User Events in a PolymerTemplate with
  examples of the usage of @ModelItem.

All changes

https://github.com/vaadin/flow/milestone/41?closed=1