New question type: Key signatures

Hello Anthony,

Here is a new question type submitted for code review and possible inclusion into the Moodle CVS: key signatures for music theory. The respondent is given a tonic and mode and prompted to enter the key signature for the corresponding tonality.

Overall, the format is vey similar to "musical intervals" contributed recently. I have reapplied to it all of Tim Hunt's comments from CONTRIB-1778 related to "intervals".

Thanks so much for your time and for your help.