Rename namespace QtWinExtras to QtWin.

As discussed on mailing list.

Change-Id: I7ef41d758730998e365d9e616118c4a7c811eebf
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>