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.
950f2c84a3