Blender Git Loki

Initial framework and basic reading of file-lists

Initial steps for T88184.

Adds a `filelist/` directory to `editors/`, containing C++ classes for
(recursively) reading directories and files into a file-list. The logic is
based on the File-Browser file-list, so that the actually executed logic is
basically the same (avoiding regressions or old issues re-appearing). But it
is entirely re-designed to work for both the asset system and the file browser.

For more info about the design, see T88184.

Also contains "peeking" the file list to count the files to be read. This is to
improve progress reporting later. However, it has some issues and I'm not sure
actually it's something we'll even need in the end, I see other ways to improve
things here.