fdo#85500 - TOOLBAR: Use insert page number icon for insert field control

Change-Id: Ibebe4ea7ba504b306a191ca0cd61cfd366577d5e
Reviewed-on: https://gerrit.libreoffice.org/12107
Reviewed-by: Matthew Francis <mjay.francis@gmail.com>
Tested-by: Matthew Francis <mjay.francis@gmail.com>