Make the keyboard analog limiter customisable.

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%.

Requires https://github.com/hrydgard/native/pull/229 if this is to be accepted.

Should hopefully take care of https://github.com/hrydgard/ppsspp/issues/6667.
The original author didn't intend for it to be used on mobile devices. I was just following suit. I can change it all to be revealed if you really want me to.