Feature request: Implement event scripts

[jbecker44]

Recalbox has a feature that enables users to write scripts to run when EmulationStation triggers certain events, such as starting or stopping a game (see https://wiki.recalbox.com/en/advanced-usage/scripts-on-emulationstation-events ). It would be really nice if EmulationStation had this functionality built-in so other devices could take advantage of it too. I can imagine it being extremely useful for, for example, kicking off an rclone script to pull the latest version of a save file from a cloud share before a game launches, and then push the new version of the save file back up to the cloud share on game exit.