Qr scanner refactor

[isage]

1. Use desaturate-min instead of average for grayscaling image for quirc, this produces way better QR decoding.
2. Rewrite downloads to use curl, because SceHttp SSL completly broken for system apps (can't set option to ignore certs on retail), plus it requires iTLS
3. Use one HEAD request to get both file size and name instead of two.
4. Start and stop qr/camera thread only when required, don't always run it in background (this also fixes race condition where qr update thread would overwrite decoded url)