The solution of not displaying candidate words in Chinese input method.

[as564335sa]

Kivy doesn't seem to be able to display input method candidates，Now there are APIs in sdl2 that can open this function，is SDL_SetHint(SDL_HINT_IME_SHOW_UI, "1") I tested that add this code to _window_sdl2.pyx,setup_window(...) and it worked. Can you add this code to the master version?