Auto priority when downloading content, loading zite

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

A simple setup could speed up the loading of content. If the user downloads a lot of data at once, the loading of the pages will slow down.

**Describe the solution you'd like**

-1 . When the user click a new content zite or file this point pause all another downloads untill the content download are finished then automatically contiune all other downloads. The point the program detect when the user currently want a content and when the user is less active the previously requested content the program downloads. So it doesn't matter how big the bandwidth is the content loading does not slow down.

-2. HTML, CSS, pictures, JS, PDF files are display the browser. Until these are downloaded, the website will not display properly. Always download this files first so zites are loading much faster. Then another files what does not need to display the website correctly can download the program when this files are finished.

-3. Download maximum 4 zite or large file and pause another files when the user not active. And when the 4 zite or large file are finished then contiune another downloads.

该 issue 没有指定文件、测试或入口点。首先定位现有的下载调度和 zite-loading 行为，然后为暂停后台下载、优先处理页面资源以及限制并发的大型下载定义测试；当所请求的优先级规则能够正常工作且不会减慢页面加载时，即表示完成。

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