sidebar: more cleanup of text panel

Change-Id: Iad9b619d599ed13e3d783ca174e7ad5f0053c622
Reviewed-on: https://gerrit.libreoffice.org/7826
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>