HIG-ify Impress remote dialog and add help button in dialog

Change-Id: I17a67090121af9fb9656a3afa177fa4c5403332b
Reviewed-on: https://gerrit.libreoffice.org/11154Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>