Distinguishing between two modes - "worksheet" & "worksheet abridged"

The former implies an in-class worksheet made by a non-indie teacher
The latter implies a takehome worksheet made by a non-indie teacher

For indie teachers, no worksheet is ever compiled. Rather
students are shown the requisite questions from the vault/ itself
Retain documt title w when creating abridged worksheets
Moved sanity checks from within \bottomQR nor \marginQR ...

... to the calling callback functions.

  printanswers        cancelspace         implies           QR-code?
  ------------        -----------         --------          --------
      false               false        worksheet / exam         yes
      false               true         abridged worksheet       no
      true                false             quiz                no
      true                true            non-sense             -
Added a comment block for \printanswers & \cancelspace interplay ...
Merge pull request #648 from abhinavc/master

Sundry changes for compiling abridged worksheets as part of indie offering