Add the Navigation buttons to the Search toolbar

They were remove from below the scrollbar before and it was decided to put them on the Search toolbar: http://nabble.documentfoundation.org/Libreoffice-ux-advise-Remove-the-Navigator-button-below-the-scrollbar-in-Writer-tp4083097p4086370.html

Change-Id: I5f7dfc0aa522e97cf3821c494525c967dcd6a92c
Reviewed-on: https://gerrit.libreoffice.org/7006
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>