Flickable doesn't react to second touchpoint while first touchpoint is dragging DragHandler

We still have tst_FlickableInterop::touchDragSliderAndFlickable() blacklisted since QTBUG-86729; this is what it's testing.

It's a behavior change since Qt 5; but whether this behavior should be the same needs a bit of consideration. Maybe we should consider it an improvement that a "stray" touch does not start the Flickable moving. It depends whether there are more useful use cases that are being prevented by this change.

In Qt 5, Flickable didn't handle touch events; so while DragHandler handled one touchpoint, a QMouseEvent would be synthesized from the other, and Flickable handled that.