Add SDL keycode mapping customizable

[Forairaaaaa]

Can customize like:

auto panel = (lgfx::Panel_sdl*)(...).getPanel();
panel->addKeyCodeMapping(SDLK_q, 0);
panel->addKeyCodeMapping(SDLK_w, 1);
panel->addKeyCodeMapping(SDLK_e, 2);
panel->addKeyCodeMapping(SDLK_r, 3);
panel->addKeyCodeMapping(SDLK_f, 4);