Convert chart legend position tabpage to widget UI

Change-Id: Iefdbbff83ea05c2e926191a2058a0be67c281789
Reviewed-on: https://gerrit.libreoffice.org/6824
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>