fdo#80651 - Add 'Single Page' button to Page Preview Toolbar

Only Oxygen and Crystal theme need another icon then the 'adddirect' one.
Both themes have a 'sum' sign in that icon. Other themes shows a nice single
page icon.

Change-Id: I7db13014f6fa97b2a9474a66a27a70352ea69b29
Reviewed-on: https://gerrit.libreoffice.org/10053
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>