Hotkey options for direct save/load states

First and foremost consider this:

• Only RetroArch bugs should be filed here. Not core bugs or game bugs
• This is not a forum or a help section, this is strictly developer oriented

Description

This is a follow-up to https://github.com/libretro/RetroArch/issues/12144. I would like to discuss the possibility of adding hotkeys for direct save and load states into RetroArch. I have added it to my own fork, and you can see what it l looks like in the image below:

Reasons I want to discuss this feature:

• As mentioned in the other issue, I run a Twitch Plays channel, and separate slots make it much easier to control permissions for individual savestates.
• Some people, including myself, prefer direct states as opposed to changing save slots then saving or loading. Other emulators have this feature as well.
• What I have now has only 6 slots present at all times. It may be better if the number of available slots were user-defined to a certain limit - say 10. There could be a menu option to enable direct saving/loading, and the menu options for each slot could be visible based on this option.
• Implementation-wise, everything is in place for this feature, so it'd be trivial to add. It leverages existing code to set the current save slot to a given number then save or load it.

I am fully willing to work on a PR for this, considering I have already done a large chunk of the work implementing it. I hope to discuss this further with you soon!

Start by reviewing the related discussion in issue #12144 and the described direct save/load implementation in the author's fork. No source files or tests are named; done would mean configurable direct save/load hotkeys, user-defined slot limits, and menu options that reflect the setting.