HandlerExtension + demo WIP.

[JohanLarsson]

• Proof-of-concept implementation for discussion.
• No optimization in the reflection.
• No filtering, draws constantly.

  Questions

• How do we want names and xaml syntax? As it is written now there is only one constructor forcing us to pass in a mapper.
• I'm thinking maybe the mapper should get sender, eventargs and the observer passed in. This would enable:
  • Passing in the observer would enable filtering in the mapper.
  • Rx style eventpattern with both sender and args.