Add timed automatic savestate

[hizzlekizzle]

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
4. RetroArch codebase follows C89 coding rules for portability across many old platforms check using C89_BUILD=1

Description

Triggers an automatic savestate to the 'auto' slot on a timer. This is an oft-requested feature, particularly for handhelds. It should also help with Android and iOS where people often kill the process instead of exiting, which renders the autosave on exit feature useless.

Related Issues

These are tangentially related: (#14518), #17612, #18025

and it directly closes #10591

Related Pull Requests

none

Reviewers

@sonninnos