This fixes the logic of split for layout toggle when the default
layout is L_TABBED or L_STACKED. When the default layout is L_TABBED
or L_STACKED, the container/workspace may not have a prev_split_layout.
This was causing L_NONE to be returned by get_layout_toggle, which was
being handled as a syntax error. This adds logic to try
config->default_orientation when prev_split_layout is L_NONE. If that
is also L_NONE, then L_VERT is used when the output is taller than
wide, otherwise, L_HORIZ is used.