Blender Git Loki

Make selection & co really working even during listing process.

This commit replaces `filelist_numfiles()` by `filelist_files_ensure()`. The later
also returns number of visible files (i.e. filtered ones), but also ensures filelist
is filetered and sorted.

Otherwise, during listing process, operators could be executed between an update of the listing job,
and the refresh from the file space - i.e. at a time filelist is virtually empty (from filebrowser PoV).

Also, got rid of 'need_refresh', this ended up being unused...