Read desktop files inside AppImages

Remaining part from https://github.com/helloSystem/launch/issues/26#issuecomment-1660001926:

```
% chmod +x ~/Downloads/Telegram_Desktop-x86_64.AppImage
% ~/Downloads/Telegram_Desktop-x86_64.AppImage --appimage-extract
% cat squashfs-root/telegram.desktop
(...)
MimeType=x-scheme-handler/tg;
(...)
```

So, when encountering an AppImage, then "the system" (I still need to decide which component) needs to place the link to `~/.local/share/launch/MIME/.../` based on the information in `MimeType=` in the `.desktop` file inside the AppImage. (This already happens for `.desktop` files which are not inside AppImages.)

Possibly we can use the new libsqsh library by @Gottox for this. It looks promising and is under a permissive license.

* https://github.com/Gottox/libsqsh/issues/34