Allow Button Remapping including second player

[ghost]

Certain Single Player games, like Celeste X require inputs from two controllers to properly play the game.

[jtothebell]

I wasn't aware of any games that require 2p controls to work, but I am planning on allowing control remapping along with customizing the cart directory and saving the last screen stretch mode. I'm going to edit the title of this issue to also include remapping other player controls.

[Check120]

Especially for the Wii U version (family console !), it will be great to have multiplayers controls. Hope you have time to add this feature one day.