ui: Add dependency handling to UserInterface.get_question

ui: Add dependency handling to UserInterface.get_question

We start the stack with all the leaf questions (i.e. questions that
are not dependencies of other question).  If the user gets one wrong,
we push the question back on the stack (so they can try again later),
and also push all of that questions direct dependencies onto the stack
(so they can get the background they need to answer the question they
got wrong).