sdl2 insert/delete and F14

[Mete Balci]

Keyboard mapping for Insert and Delete in sdl2 backend were not working. The problem is sdl2.c:sdl2_keysym_to_xk does not have:

case SDLK_INSERT: return XK_Insert;

case SDLK_DELETE: return XK_Delete;

cases. and also while looking at this, I saw another case with a typo:

case SDLK_F14: return XK_F13