Issue should get deleted when corresponding marker is deleted.

We currently have the ability to delete issues from the Issue Explorer, which in turn deletes the
corresponding markers; however, it should also work in the other direction. If a marker is deleted, the corresponding issue should get deleted. For example, this would happen
if the user deletes a line of code containing a marker. In this case the marker would be deleted, and we'd expect the corresponding issue to be deleted and removed from issue explorer.