Support defining custom elements via a decorator

[matt-gadd]

Enhancement
The ergonomics for creating a custom element from a dojo widget are currently a bit fiddly via the customElementDescriptor, it would be nice if we could support a simplified form as a decorator on the component.

As part of the simplification we'd like to drop or do better with dealing with things automatically for the author. that would mean that they would no longer have to specify a mapping for anything. events would automatically drop the on and be lowercased? we would no longer want to support changing the property names or attribute names either I don't think as so far it's caused more trouble than it's worth?

Something like:

@customElement({
  tag: 'my-foo',
  properties: [ 'foo', 'bar' ],
  attributes: [ 'baz', 'qux' ],
  events: [ 'onSomething' ]
})
class Foo extends WidgetBase {
}

The only potential last thing we'd need is still functionality for the initialization function.