Separate worker thread to support slow filesystems#15

As Adam reported here,

https://savolai.net/software/jpeg-autorotate/#comment-18061

Jpeg Autorotate 3 user interface apparently becomes unresponsive if filesystem is slow.

Moving filesystem operations to separate thread to mitigate can help. This improvement is looking for funding, as mentioned in the above discussion linked. Please email me at olli @ jpegrotate .com if you want to support this.