This is a bit of a red herring, since WMTree is only used in wmmenugen, which I
don't think I've ever directly used. See notes at the top of tree.rs for more
musings on whether this should be in wutil-rs at all.
WMTreeWalk is only called once, with depthFirst set to true, so we might as well
just hard-code that behavior.
Also, this parameter appears to have been misnamed (since the search is always DFS)
and should properly be named to indicate that it controls if this is a pre- or
post-order traversal.
This is a first step towards migrating WINGs/tree.c to Rust.
Happy finding: no parent pointer are actually needed, so we don't have to use a
shared structure for node pointers!
