Makes it possible to use non-trivial C++ types in `FileListEntryCache`.
I'm planning to use this for blender/blender!130543, to store status
information about downloaded previews. While it's possible to use
C-style types instead, this is easier and a worthwhile general
improvement.
I turned `FileList.filelist_cache` into an allocated pointer, so
`FileList` doesn't have to be ported to be a non-trivial type as well.
We probably want to do that too at some point.
Pull Request: https://projects.blender.org/blender/blender/pulls/141411
183420bd07