Make FeedMenuItem layout consistent in Cinnamon 1.8.x

- Layout of underlying Applet.MenuItem has changed in Cinnamon 2.0 (now
  uses a table). Since the implementation of Applet.MenuItem is trivial
  and we need to make the layout consistent I have re-implemented
  FeedMenuItem as a direct subclass of PopupBaseMenuItem instead
- This isolates use from the layout changes in 1.8.x -> 2.0.x