Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.
Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.
Now that the unused DocDump was removed, the `editor/doc` subfolder is
redundant.
Similarly, there's no reason for Collada to have a subfolder for itself
when glTF or OBJ don't.