Blender Git Loki

SpaceFile: Refactor sorting and filtering of filelist.

New code shall be more easy to maintain and extend.
Sorting is now handled quite the same as filtering, and all filtering parameters
are now packed into a sub-struct to help extending it later.

Also done some optimizations in filelist refresh, and sorting/filtering area.
Now we should avoid re-sorting and re-filtering too often, also removed
calls to those in read_xxx funcs.

Note thumbnail job is still started basically on each call to `file_refresh()`,
will be addressed in next commit.