Blender Git Loki

FileBrowser: internal listing: use own, much lighter struct to store whole list.

Since with internal listing we have no choice but to list everything, store
those data in the smallest possible struct - note we do not even store preview
here, these are only handled in full FileDirEntry cache (re-loading them from
cached thumbnails is really quick anyway).

Still not complete, have to re-do ID previews now, among other things...
And some memleaks again with previews, ugh.