Further optimize handling of torrents with large number of files

[equeim]

1. Don't recursively calculate properties of a directory every time they are accessed, do it only the first time and store them, and update them when they change
2. Don't use polymorphism for TorrentFilesModelEntry. Instead store file or directory data in it directory using std::variant. This allows storing entries in contiguous vectors in directories instead of allocating each one separately and storing pointers.
3. Don't split file paths into std::vector each time they are parsed from JSON, do it only once when building file tree.