Group like Impl* functions in outdev.hxx

Change-Id: Iede8395d9287a857601080e7a65dc3a3eea93c9e
Reviewed-on: https://gerrit.libreoffice.org/7890
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>