Improved backtracking navigation.

The change involves a corner case. Consider the top-level command (m) which
lets the user view all materials systemwide. No score is currently active. But
what should happen when the user enters (score)?

OLD: score manager returned user to home. This made (score) function like
(home) in just this one case.

NEW: score manager does nothing when user enters (score). This reinforces the
"location" of user experience.