fdo#85594 moving merge cells to standard toolbar

Change-Id: I73b199458e23ffafc269c9683ca30ec87b41f770
Reviewed-on: https://gerrit.libreoffice.org/12417
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>