Added support for saving and loading directly into savestate slots 0 through 9

[kimimaru4000]

Guidelines

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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
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Description

This is a cleaner PR for #13024, as it's been many years since that was last opened. All the changes have been updated and are in one commit.

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

• 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

Related Pull Requests

[Any other PRs from related repositories that might be needed for this pull request to work]

Reviewers

@LibretroAdmin