Add version trait to Widget base,

From widget Trello: version numbers for widget models;
when reloaded, will give the widget a chance to do
something intelligent or fail (by default) if the
version number doesn't match.

https://trello.com/c/joYVGd4d/49-version-numbers-for-widget-models-when-reloaded-will-give-the-widget-a-chance-to-do-something-intelligent-or-fail-by-default-if-