Add EmptyElementFilter

[NebelNidas]

This is an adapter which filters out elements that effectively don't contain any data. Entries are only forwarded if they contain:

1. a non-null comment,
2. a non-null or non-equal destination name,
3. a non-null or non-equal destination descriptor, or
4. a child element to which one of these points applies.

[modmuss50]

Looks good, great tests.

[Player3324]

Relying on the visit passes to have the same order seems a bit fragile and the 2 pass approach seems unnecessary?

I believe all it'd have to hold onto is a stack to record the src data and a pass-through-grant boolean for the parents and their parents, giving the data retention abilities of recursion to the iterative visitation.

If that stack's elements are a mutable class with a few fields to hold a resetable kind, src name, desc, arg index etc it would only have to allocate max(stack_depth) objects for the entire pass.

[NebelNidas]

Done. I used the two-pass design first because it's harder to mess things up that way, but with all the tests I added I think the new version I just pushed works just as fine.

[Player3324]

I see you ended up flattening it, bu that's fine I think

Thanks, looks good from a cursory glance