Slider: pressing the handle should not change the slider position

We have to maintain the handle offset until the drag operation
has ended. If the user clicks outside the handle, we center
on the cursor position as before.

Task-number: QTBUG-31042
Change-Id: I8f23d998774278c6353d31b6fffea54596f7158e
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>