This replaces two members of `BlendfileLinkAppendContext`:
* `LinkNodePair libraries` -> `Vector<BlendfileLinkAppendContextLibrary *>`
* `LinkNodePair items` -> `std::list<BlendfileLinkAppendContextItem *>`
This simplifies the code quite a bit.
Making `libraries` a `Vector` works because afaik we never iterate over the
libraries while also adding new ones (which would invalidate iterators). It's
also nice, necause we use random access in some cases.
`items` can't become a vector, because afaik more items may be added while
iterating over it. So it's still a linked list now, but with a better type that reduces
boilerplate a lot.
Pull Request: https://projects.blender.org/blender/blender/pulls/124524
83373002d2