This resolves an irritating issue in the code editor where SDL2-specific functions could't be found. Not entirely sure about the SDL include file logic for other operating systems than Linux so the #ifdef directives may need to be updated further at a later date.
* add hotkey button gfx and hotkey to input configuration
* prompt user if no hotkey set, and offer to use select
* if no hotkey is set, still save an entry with key id 0, for backward compatibility