Submenu experiment WIP
Change FeedTitleItem class into generic feed display class

- Started splitting the menu building into class methods so it can be
  updated separately
Feed reader as a child of FeedDisplayMenuItem

- Each FeedDisplayMenuItem now completely manages a single feed
  including handling update and error callbacks and supplying the owner
  applet with summary info (title and unread_count)
- This amounts to a significant refactor of the applet code, but will
  hopefully prove more maintainable going forward
Make sure feed display params are set before creating FeedMenuDisplayItems
First implemenation of "accordion" effect

- Auto-open first feed submenu
- Close other submenus when a submenu is opened