fdo#85937 Add 'Close Sidebar' command to Sidebar menu

Change-Id: I83248398a9a8f2a8abb4e10d7ef9f1c203dddbf5
Reviewed-on: https://gerrit.libreoffice.org/13297
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>