Use setState() instead of forceUpdate() to update details view when item content is loaded

 * Avoid rendering the details view when showing the main item list

 * Use a more idiomatic way to update the details view when
   an item's content is retrieved by using setState() instead
   of forceUpdate()
Add a transition between details view and item view

Slide details view in from left
Fix update loop when fetching item content

DetailsView was triggering a content fetch
which triggered an update every time that
an update completed.