Support saving and loading directly into states 0 through 9

[kimimaru4000]

Guidelines

1. Rebase before opening a pull request
2. If you are sending several unrelated fixes or features, use a branch and a separate pull request for each
3. If possible try squashing everything in a single commit. This is particularly beneficial in the case of feature merges since it allows easy bisecting when a problem arises

Description

This PR adds new hotkeys for directly saving and loading into save states. More details:

• The savestate_max_direct_slot setting determines the highest slot that can be used with the hotkeys. For example, if this value is 1, hotkeys for directly saving and loading slots 0 and 1 can be used. This defaults to -1, which disables the feature.
• Similarly, the options and bindings for the direct slots won't show up in the Hotkeys menu if the slot is greater than savestate_max_direct_slot.
• None of the new hotkeys are binded by default.

Related Issues

#12990

Reviewers

@jdgleaver @twinaphex @bozothegeek

[inactive123]

Hi, we appreciate the PR certainly.

However, we have some runloop changes planned ahead and we might have to significantly rework it. In the process, when we get to it, we'd also like to reimplement this idea ourselves so that it wouldn't hurt performance in the process (all the extra conditionals added to the runloop here kinda concern us in its current state).

So hopefully you'll understand that we want to freeze this PR until we get to this runloop state refactor. We thank your for your understanding.

[kimimaru4000]

That's completely understandable; thanks for the update! Do you have more information on the runloop changes and the roadmap for it?

[kimimaru4000]

@twinaphex Do you happen to have any updates on the roadmap for the runloop changes?

[kimimaru4000]

@LibretroAdmin Hi, I've updated this PR with the latest on master after a long while. Can you have a look please?