Add pregrading

This MR adds the pregrading feature.

Other things fixed in this MR:

• The way feedback is inserted has been changed when a MultipleChoiceOption is inserted. Previously, the FeedbackOption that was added with the MultipleChoiceOption was blank, now FeedbackOption.Text is automatically set to MultipleChoiceOption.Label
• A function was added that infers the 4th corner marker if only 3 are detected.

Closes #24 (closed).