Fix decrementing tasks left for non-task exercises

[otacke]

If the Column.isTask function does not report a content type to be a task (usually scorable), but the content type sends out xAPI statements, then Interactive Book may decrease the number of tasks left by 1 and this leads to a wrong count of interactions on the summary screen.

This can, for instance, happen for Agamotto that is not considered to be a task (not scored), but sends out xAPI completed once all images have been shown.

When merged in, the number of tasks left will only be reduced by 1 if the instance at hand is considered to be a task.