Set web component as properties, not attributes [breaking change]

[wwwillchen]

Fixes #730.

Breaking change: Web components are now set via DOM properties and not attributes. This means you can not use Lit's attribute API, e.g.

  static properties = {
    value: {type: Number},
    incrementEventHandlerId: {attribute: 'increment-event', type: String},
  };

I mixed up DOM attributes and DOM properties and only primitive types, e.g. string, number, boolean, worked when I erroneously set the web component properties as DOM attributes. Lit's default converter worked for the primitive types.