[ Feature Request ] Export/Import identy/s with qrcode

As the mobile version is spreading, I think that it can be good to have a more userfriendly way to import/export private key/s of the identies.

So, this is the request, I think that ZeroNet should show the qrcode of the private keys of the identies.

On this way the user from the mobile app, will be able to import the desktop identity/s by simply scanning it.

@canewsin Also this is required on the mobile app (currently missing), I mean, it should be easy to import/export them by simply scanning them.

未提及任何文件、测试或入口点。首先定位 ZeroNet 的身份私钥导入/导出流程以及移动应用的身份处理方式，然后确定一种支持桌面端与移动端之间扫描的 QR 表示形式。完成标准是：身份可以通过 QR 码导出和导入，且不会暴露不兼容或不完整的密钥数据。

由索引模型根据 Issue 内容生成。