Implement iconic live previews/iconic thumbnail icons.

[ChangeLog][QWinThumbnailToolBar] Added pixmap properties
and change notifications for thumbnail icons and live
preview.

Task-number: QTBUG-36730
Change-Id: I31ad0ff347235bc7933c1c14dece359ec0c36b39
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>