Drop autoscroll feature

This feature has been without maintenance since approximately 2014, and adds some complexity to the already hard-to-follow scroll logic in PpsView. In addition, there's been some feedback that most people are not even aware of the existence of this feature. So drop it completely. Some research has showed that some previous use might be related to reading musical PDFs without having to do a full gesture. However, we can most likely figure out whichever use-cases are needed with alternative solutions, without having to cram more functionality into PpsView.