SDL2 backend: Move from scancodes to keycodes
This change is required in order to support non-English keyboard layouts. Currently
orbclient uses the US-QWERTY layout for every keyboard instead of the system defined one.
Reasons for this change from the SDL documentation:
The main difference between these two values comes into play in situations where the operating system is mapping the physical keyboard keys to different virtual letters. For instance, when a standard QWERTY keyboard is mapped to a German QWERTZ layout, the "Y" key will generate events with a scancode of SDL_SCANCODE_Y, but a keycode of SDLK_z. Which one to use is left to the application: scancodes are suited in situations where controls are layout-dependent (eg. the "WASD" keys as left-handed arrow keys), whereas keycodes are better suited to situations where controls are character-dependent (eg. the "I" key for Inventory).