QTI metadata: add expected grading time

In the question pool authors can already enter the typical learning time. This is handy when planning a test and finding items that fit into a certain time frame. 

When a test contains open test items that need to be graded manually however no such support is available for the graders. Typically graders are payed by the minutes they spend with their grading activity or better said: grading a test normally has a certain value based on the minutes typically spent for grading (regardless of the actual time the grader spent). 

The goal of this issue is to add optional metadata to QTI test items where the test author can define the typical grading time requires for manually grading the item, or when used in reports, the time that is accounted when a grader corrects this question.