Small fix to TentativeRequest.

Refactoring broke TentativeRequest, so fixing.
Separating FragmentScreen & DialogScreen

It turns out DialogFragments cannot be used in place of Fragments in all
cases (DialogFragments "cannot be attached to a container"). Instead,
we'll separate them out so that NavigationFragment can soon become a
FragmentScreen.
Refactoring NavagationFragment

Split into NavigationController & DrawerScreen to better match with new
paradigm.