Allow users to abort a long-running query

[KCMertens]

This will need built in support from BlackLab. It might be fairly simple (list to some event, interrupt some thread?), but needs investigation.

[jan-niestadt]

It's worth considering, but it shouldn't be possible to cancel a query that someone else is also waiting for. This may seem unlikely, but if you have e.g. a class of students all working on the same assignment, it could be a problem.