Add autosave/autoload feature

[goweiwen]

Introduction

This PR adds an autosave/autoload feature and makes it the default behaviour:

• When quitting a game, automatically saves a state to slot 9.
• When loading a rom, automatically loads from slot 9 if state exists.

Some changes are required to the picoarch submodule. If you plan to merge this, I suggest forking picoarch and merging those changes as well.

Details

Main Menu

• "A" key prompt will now show either "Continue" or "Start" depending on whether an autosave is present.
• Remove "POWER: Sleep" prompt, moved "B: Back" to the left.
• Remove "X: Resume" button as it's no longer needed.

mmenu

• When selecting "Quit", you can now press "X" to restart the game. This is necessary since it is no longer possible to cold boot without going into the advanced menu.
• In mmenu, "Quit" is renamed to "Save & Quit" when save states are allowed.
• Remove "POWER: Sleep" prompt, moved "B: Back" to the left.

picoarch

• FAKE-08 save states are disabled. They don't work and break the autoload feature.
• When quitting, autosaves to slot 9.

[Xpndable]

This will be tested soon, now that v1.0 is close to release, thank you for your efforts 😊

[anzz1]

This should definitely be configurable, either by an advanced setting or a compile-time flag.

As this is not a case of a "missing feature", but rather a conscious design decision. It wasn't that when designing MiniUI the autosave/load couldn't be added (as is apparent from autosave/load existing when powering off while in-game), but that it shouldn't be added.

Quite probably many fans of MiniUI appreciate this design decision, where Quit means Quit and Save means Save, and A cold boots a game while X resumes from last save if available. I wouldn't reverse that design decision lightly, at very least a big design change like this should be made configurable and not forced upon anyone.

Obviously there are also people who dislike the current design and would prefer the automatic saving/loading instead, so a choice between the two wouldn't be a bad idea.

[goweiwen]

I agree, and this PR touches too many core components to be merged regardless. Closing this for now, thanks.