fdo-50672 Escape underscores in menus for unity

Change-Id: Ibb4647c1ff6c2858fea888efae975e8e5c5011e2
Reviewed-on: https://gerrit.libreoffice.org/8773
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>