Generic Vaadin8 field for converted values

In Vaadin8 the conversion between the model data type and the field is done in the Binder. This prevents automatic mapping for the models as some manual step must be done beforehand. But since writing custom fields now is way more reasonable, here is a first draft for a generic converter field. Just provide the backing Field and an Converter.

https://gist.github.com/christoph-frick/947fec59cfd09f7140933b52102cc041