fdo#69878: Add [sub|super]script entry to text style context submenu

Make Impress consistent with Writer in this regard

Change-Id: Ib01472d88c20f05a375b57d8a79576fc447cc250
Reviewed-on: https://gerrit.libreoffice.org/8684
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>