User setting available on prototype phase

[Linver]

Hi. One of my users ask me to make an option to change the main menu background image, selectable in user/player setting.
This request come from the fact that Factorio force the users to syncronze some settings on connection to one server, the problem is that ambient background musics and main menu background image are properties that don't need to be the same in different maps/server configuartions, because don't affect anything that users share in multiplayer, they influence only ambient music played for a users (that is not synchronized, or shouldn't) and the image showed in the main manu.
This properties can't be customized (from user to user) in the actual setting mod interface, because the access to the user setting is denied in data stages, players need to have the same setting (for things doesn't affect the game consistency) of the server where they are connected.

[Rseding91]

If the values are readable in the data stage they have to be identical because any mod can read those values and change their behavior based off of them.

Additionally, all loaded sounds are readable runtime so they also need to be identical.