Sidebar Text panel cleanup

Change-Id: I6cbf5a9e5ad9d61df2abbcc835ba60d45768deeb
Reviewed-on: https://gerrit.libreoffice.org/7687
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>