Controller support

[rollerozxa]

Either only for playing in adventure mode and with RC controls, or full controller support everywhere.

Steam Input of course exists but having it built-in to the game is best.

SDL2 should be able to do most of the heavy lifting for us. Unsure how well the ancient SDL2 beta version that the Android version uses works with controllers, but maybe will be desktop-only for now.

(For reference so I don't forget about it: a colleague over at Minetest has a blog article about setting up SDL_GameController: https://blog.rubenwardy.com/2023/01/24/using_sdl_gamecontroller/)