to avoid doing a read-modify-write cycle on the item images on the
toolbar, add an overlay image member to Toolbox items.

part of the process of making Bitmap an internal detail of vcl/

Change-Id: Ie4a886c48484a06694fc4c066ee0845b39d27f0b
Reviewed-on: https://gerrit.libreoffice.org/49649Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>