Activity stack traversal now working for feeds/folders and entries lists

FIXME:
- Occasional CBL views crashing when moving through activity stack.
- Refactor/clean existing view adapters (don't recreate, i.e).

Next features in order:
- Open entry link in external browser.
- Long touch menu: mark as read/unread.
- Press star icon to mark as starred.
- Refresh current folder/feed list or entry list* via menu and/or app bar
  icon/button.
- Actual entry (web?) view including title, summary, content...

* State of Android-compatible Java RSS+Atom parser libraries isn't looking
  good. Need to investigate more.