QStyleHints::startDragDistance is used as the standard drag or flick
threshold in QtQuick generally.  We should follow suit because it
may depend on platform characteristics such as screen resolution and
sensitivity of the input devices.

Change-Id: Ibbc7b4549351f795c10de792cb8fa3ff9f5ae58d
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>