Slider: fix behavior inside a flickable on touch

Do not move the handle immediately on press, but delay it until the
drag is over the threshold. This makes Slider behave inside a flickable
in a sensible way ie. the handle is not prematurely moved when flicking

Change-Id: Ie02762717156e75be9db757330a95a3f50451f1a
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>