(Redux) Make the keyboard analog limiter customisable.

This got merged accidentally before it was truly ready. I'm sorry about that. So let's continue discussing it here. Prior discussion at https://github.com/hrydgard/ppsspp/pull/6727.

Some games use differing values for deciding when to run/walk/crawl/etc., so it makes sense to be able to use different values for them, instead of always assuming a deadzone of 50%.

Should hopefully take care of #6667.