Answer dialog no longer accepts blank answer. (Issue #64)

The answer dialog no longer accepts a blank answer as valid. It will now
display a new dialog if user attempts to submit a blank answer.
Fixed issue with score by round chart using a different font than other
plots. (Issue #56)
Implemented hiding of duplicate answers. (Issue #4)

Added a "duplicate" status for answers.
Implemented option to hide answers marked as duplicate (default true).
(Issue #4)