bugfix: forgot to mark split containers as split = true (+test) (Thanks szalik)

bugfix: forgot to mark split containers as split = true (+test) (Thanks szalik)

When the workspace layout (formerly orientation) was forced to change
due to a move command, the split container we created was not marked as
split = true, which caused tree_flatten() to errnously kill the contents
of it and thus one window ended up unmanaged.

Also, the logic in tree_flatten() was inverted due to commit de94f6da.

fixes #790