Blender Git Loki

FileBrowser: First step towards allowing partial "storage" of entries.

That code is currently pretty stupid, but we need to clearly separate
what is handled by filebrowser itself, and what is by listing code
(be it internal one or future asset engines).

Plan/idea is to have a dual cache system (one 'block' of entries - centered
on currently viewed ones -, and one 'random' FIFO for random access to any entry).

This implies filebrowser itself shall now only be aware of the number of (filtered)
entries, and it then requests entries as needed.

Also, made size of columns in filebrowser fixed (ultimately, user configurable),
since it has no more access to the whole list of entries!

Still heavy WIP.