Fix process priority

[JacobDev1]

Hi! This is a very nice program. Below are 2 small fixes.

I fixed the niceness values on POSIX. They were reversed originally.

I removed the REALTIME priority from the UI on Windows.

You should almost never use REALTIME_PRIORITY_CLASS, because this interrupts system threads that manage mouse input, keyboard input, and background disk flushing.

~Windows docs

This priority also requires SeIncreaseBasePriorityPrivilege which users typically don't have. ~docs

Sidenote: Negative niceness requires elevated privileges on POSIX so these options remain non-functional.

[cdgriffith]

Thank you for the fixes!