Support for master solution in QTI essay type in word export

Currently the QTI editor allows adding of a master solution only when the solution should be displayed to students. However, this only makes sense for online tests. When exporting tests with the Word export the master solution would be needed by the authors who have to correct the essay question.

The following should be implemented:

• For essay types, always show the "correct solution" field regardless the "Show correct solution" checkbox (only for essay types, not needed for other types)
• In QTI runtime, show correct solution to students when checked (no change needed here), when not checked do not show solution even though a solution is entered in the editor.
• In the QTI word export print correct solution below text input area in the *_responses.doc file. Never print the correct solution in the students version of the word export