auto-scroll triggers off the cursor nearing an edge

* fix #3356
* not sure what I was thinking doing scroll just based on widget being outside, you also need to check mouse cursor moving towards the top/bottom (~10px) and being close to edges (8% min 30px) to start scrolling.
* also sped up scrolling a bit now

this makes it possible to horizontally move large objects without scholling to top/bottom unless you mean to

Author: adumesny