Fix QWinThumbnailToolBar::setButtons()
Fix QWinThumbnailToolBar::setButtons()

It was inserting buttons twice to the internal list.

Change-Id: I62ffc2f50c0a13624f30ef5bc6ca6389221a0a5e
Reviewed-by: Friedemann Kleint <1b9f2592239daaec4856ccc1cfb4d2d55b35fb71@digia.com>

kobolabs/qtwinextras,kobolabs/qtwinextras