Convert chart data labels tabpage to .ui

Change-Id: I6ffd992fe8a2cb290c8f8992922f0e54276e6ae1
Reviewed-on: https://gerrit.libreoffice.org/7124
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>