pointer handlers "mixer" manual test: slider knobs still come out of their grooves

It's an old and well-known problem at this point, and we've solved it several times, but none of the recent solutions ever got reviewed and committed. It's easy to see when you make circular motions with multiple fingers: sometimes the sliders are dragging, sometimes the Flickable grabs, and sometimes when that happens, the DragHandler's vertical constraints are broken.

https://codereview.qt-project.org/#/c/207463/ might help.

https://codereview.qt-project.org/#/c/209816/ might help, but might also break more complex use cases.

https://codereview.qt-project.org/#/c/191786/ might help and has been sitting unreviewed since April.

It might be good to have two solutions: 1) the DragHandler has to be sure to use the right coordinate system. That's the main thing, because otherwise there will be bugs in other use cases. 2) if it's also true that the DragHandler should not steal the grab back after it's already been taken, that's a secondary fix which will mask the original bug.