make it a custom widget so it is accessible e.g. to sidebar panels

Change-Id: Ic36a9a8af96a09fc76efd8e9ae75b8ebdf81717e
Reviewed-on: https://gerrit.libreoffice.org/25764Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>