Sharpened logic for when QR code should be inserted in the bottom-left corner

QR codes should go in the bottom left in the following two cases:
  1. for the first sub-part on a page
  2. for the first sub-part AFTER a margin-figure (there is usually just one per question)

However, if the bottom-left corner is taken, then \embedQR should do nothing
Merge pull request #602 from abhinavc/master

Sharpened logic for inserting QR code in the bottom left corner