Turn other Activities into Fragments

- Things launched from the Nav menu are now Fragments that replace each
  other in the PageActivity container, and create a backstack in the
  FragmentManager.
- Made full-text search into a Fragment
- Made the PageActivity have a real ActionBar (to be built out in the next
  patch), into which the actions of the fragments are added.
- Made sure that savedInstanceState preserves the order of the fragment
  backstack.
- Login activity to be dealt with in another patch...

Change-Id: If9c2c11014745e75f3dbde86a724d555470c8f61