Nannanko/handle all type media file

Checklist | 检查清单

Before marking this merge request as Ready, the following must be done:

在把这个合并请求标记成就绪之前，这些东西必须完成：

• The code must compile. Preferably, each commit should compile.

  代码必须能编译。最好每个提交都能编译。

• [n/a] Any added UI text must be translated via l10n.get(). The messages must be translated into every core language: en, cmn.

  任何添加的 UI 文字都要用 l10n.get() 翻译。 讯息必须翻译成所有核心语言：en，cmn。

  • [n/a] The merge request creator must translate each message into one of the core languages. The others can be handled by others.

    开合并请求的人必须把每条讯息翻译成一种核心语言。 别的可以给别人做。

• If the merge request contains a user-visible change, it must have a changelog.

  如果这个合并请求包括对用户可见的更改，必须写个更改记录。

Handle all types media file | 处理所有类型的媒体文件

Currently, Kazv can only handle image files correctly (such as caching and downloading for encrypted file), and the relevant QML code is entirely in Image.qml.

目前 Kazv 仅能正确处理图片文件（比如加密文件的缓存与下载），并且相关的代码均直接写进了 Image.qml 中。

I will abstract the relevant code into a separate code file for reuse for all types of media file messages.

我将试着将相关代码抽象为单独的代码文件，以供所有的媒体类型文件复用。

Problems | 存在的问题

For reasons of my ability and not wanting to delay MR indefinitely, there are currently the following known issues with the features added to Kazv (left for subsequent MRs to resolve)

鉴于我个人能力问题和不想把 MR 一直拖着，现在加在 Kazv 的功能里还有下面这些问题（就留给后续的 MR 解决了）

1. Repeating progress bar. The system level progress bar and my own implementation of the progress bar exist at the same time.

   重复的进度条。系统级的进度条和我自己实现的进度条同时存在。

2. Media files are opened using an external program. If a file is not cached before opening, it cannot be opened automatically after caching is complete.

   使用外部程序打开文件。如果打开文件时文件还没有被缓存完成，那么文件不能在被缓存完成后自动打开在外部程序里。

3. Unsatisfactory code quality. I would love to revisit my code before too long.

   很希望未来能重新审视自己的代码。