add "Guess the Answer" content type to Quiz (question set)

The "Guess the Answer" content type is very well suited for self-assessment. However, it is currently limited to a single question only making it not very useful (see also the many comments here

Adding the Guess the Answer content type as an option within Quiz (question set) would allow creating a (randomized) set of Guess the Answer questions for self assessment. 

Content types: 
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