consider moving dragThreshold property from QQPointerHandler to QQMultiPointHandler; qreal?

Only DragHandler and PinchHandler need it so far. But the property is in the base class, since https://codereview.qt-project.org/c/qt/qtdeclarative/+/267206 DragHandler used to be a single-point handler, but not by that time.

There could be a single-point handler that needs a threshold though; so this is questionable.

Also the type is int... I forgot if there's a good reason API-wise, but it's an oddity when most distance-measuring properties are qreal. Implementation-wise, it's a qint16 for better packing; that's nearly a moot point, and the "packing" is currently mixing signed and unsigned types, which won't work anyway.