This patch adds a "clear" button to all the questions in a quiz that erases any answer made in this question.

I created this because there are some questions that cannot be unanswered, in example the true/false questions, so when the alumn has answered it he can't undo this and is bound to answer one option.

The patch affects to the question/engine/renderer.php file, it works with a javascript code that, I suppose, does not go there, so someone who knows moodle better than me should put it in the right place.