FT_APPENDLAYOUT concatenates multiple layout descriptions that have been constructed
separately.
Use as
combined = ft_appendlayout(cfg, layout1, layout2, ...)
where the input layouts result from FT_PREPARE_LAYOUT and the configuration
should contain
cfg.direction = string, 'horizontal' or 'vertical' (default = 'horizontal')
cfg.align = string, 'center', 'left', 'right', 'top' or 'bottom' (default = 'center')
cfg.distance = number, distance between layouts (default is automatic)
See also FT_PREPARE_LAYOUT, FT_LAYOUTPLOT, FT_APPENDSENS