+ADD File menu to easier download ZeroNet large optional files

+ADD File menu to download ZeroNet files in the right menu and another in files menu too. This works like a seed button for advanced users. When a user want download a file, simple copy and paste the file URL. And press the download button. If it is a valid file a green check mark appears. If not a red exclamation mark with a error msg. The same setup what use all download managers. The user copy the link, paste to the +ADD File menu to the link. It's always easily accessible. No problem if it is not integrated on the site the +Seed button. If the zite have Optional Files this option are shows too. Like the "Help distribute added optional files" menu. All browser have a good Copy/Paste option. And it's works fine with 0Net large files.

The [Popup window to resumume large optional files download from browser request](https://github.com/HelloZeroNet/ZeroNet/issues/2041) would be ideal for less experienced users.

[A download manager pause, contiune download button option It would be useful](https://github.com/HelloZeroNet/ZeroNet/issues/1910) in Zero Hello files menu.This would make the downloads easy to manage.

And [Allow auto download optional files too when the zite are loading](https://github.com/HelloZeroNet/ZeroNet/issues/2243) it would make things a lot easier for less experienced users.

この issue にはファイル、テスト、エントリーポイントが記載されていません。まず既存の File メニュー、+Seed ボタン、オプションファイルのダウンロードフローを追跡し、その後、貼り付け、検証、ダウンロード管理について要求されている動作をメンテナーと明確にしてください。完了条件は、有効なファイル URL と無効なファイル URL の両方で動作する、合意済みの実装ができることです。

索引モデルが issue の本文から書いたものです。