Add a new SDL_KEYMAPCHANGED SDL event to abstract notification of keyboard layout or input language changes.

src/video/windows/SDL_windowsevents.c

@@ -626,6 +626,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
     case WM_INPUTLANGCHANGE:
         {
             WIN_UpdateKeymap();
+            SDL_SendKeymapChangedEvent();
         }
         returnCode = 1;
         break;