Popup window to resumume large optional files download from browser request

**Is your feature request related to a problem? Please describe.**

Not easy or some times impossible to download optional large files from zites without seed button.

**Describe the solution you'd like**

A simple DOWNLOAD FILE popup when the opened page include optional file. Example embed HTML5 files and etc. This fuction works like the seed button and resume the download if nobody seed the current time the file.

**Describe alternatives you've considered**

ZeroNet currently use the +SEED button. This is not integrated by many website owners. And not easy to integrate. Download managers are failed! A popup window what give you the option to resume the file download are easy to use for every user!

**Additional context**

+SEED button not integrated by many website owners. Download managers are failed! if the users try download this files whit them. Currently there is no easy good working alternative. And larger files with thie current setup are all failed.

首先检查现有的 +SEED 流程，以及浏览器请求如何处理可选的大文件。定义 DOWNLOAD FILE 弹窗应如何提供可恢复的下载，并确定当前没有 peer 正在 seeding 时所需的行为；该 issue 未指定具体的文件或测试。

由索引模型根据 Issue 内容生成。