Games: Add setting to disable autosave

[garbear]

How autosave works:

If a game supports serialization, it is autosaved alongside the rom (e.g. Mario.smc creates Mario.sav and Mario.xml).

This happens when the game is closed. It also happens every 10 seconds in case of power outage. When the game is resumed, the last savestate is loaded.

This PR adds a setting to disable autosave.

Also includes:

• refactor to avoid polluting settings source folder
• a missing forward-declaration that was interfering with code completion

Motivation and Context

Reported in March: https://forum.kodi.tv/showthread.php?tid=173361&pid=2709320#pid2709320

How Has This Been Tested?

Tested on Linux x64 with several GBA cores. When disabled, no autosaves are created or loaded.

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

[garbear]

jenkins build this please

[garbear]

jenkins build this please