Some refactoring to get LightboxInterface closer to a proper UI component

* remove reference to MultimediaViewer
* move hash handling to MultimediaViewer
* make it inherit from Element, remove code duplication
* cleaner event handling life cycle (register on attach,
  unregister on unattach)

Change-Id: Ida8f68dead758a6ae3c429eb85c548af61e46801
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/178