[SDL2] Use new wheel events in mouse handling.

The mouse wheel is not a button anymore. The old button codes and events are now
removed and we use SDL_MouseWheelEvent to get the updates.

SDL_EventState( SDL_MOUSEWHEEL, SDL_ENABLE ) is called at initialization but I
do not know it is required.
[SDL2] Update keyboard handling with SDL 2 API.

Character input is obtained with SDL_TextInputEvent. Key down events are not
used anymore. We used to carry the key code alongside with the characters. It is
not possible anymore so the key code in our key_info will be zero when the info
is about a character.

SDL_GetKeyState() is renamed SDL_GetKeyboardState().
SDL_EnableKeyRepeat() is gone.
SDL_EnableUNICODE() is gone.

SDLMod is now SDL_KeyMod.

The folowing key codes have been renamed:

SDLK_KP[0-9] -> SDL_KP_[0-9]
SDLK_[LR]SUPER -> SDL_[LR]GUI
SDLK_PRINT -> SDLK_PRINKSCREEN
SDLK_NUMLOCK -> SDLK_NUMLOCKCLEAR
SDLK_SCROLLOCK -> SDLK_SCROLLLOCK
SDLK_BREAK is gone.
[SDL2] Mouse position is adjusted using the size of the focused window.

SDL_GetVideoSurface() does not exist anymore. We get the active window with
SDL_GetMouseFocus() then we request its size with SDL_GetWindowSize().