Add default to low priority within settings

[Kaya Hakoyu]

Usually there is no effect in lighter files.

However, when compressing large files, it will cause a long time lag.

(e.g. difficult to move the mouse)

No problem if low priority mode is turned on and it also does not affect the compression time much.

But it's troublesome to click on the option to set low priority in the compression screen every time.

So it's a good idea to add an option to the settings that keeps the priority low.

Thank you for providing such a great compression software!