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.
Well, it won't even compile as is; `EmuScreen::setVKeyAnalogX()` is used on mobile.

Personally, I think it makes sense to expose to mobile, but I don't feel strongly about it.  I do feel strongly that we want it to compile on mobile devices.

-[Unknown]