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

include/SDL_events.h

@@ -94,6 +94,9 @@ typedef enum
     SDL_KEYUP,                  /**< Key released */
     SDL_TEXTEDITING,            /**< Keyboard text editing (composition) */
     SDL_TEXTINPUT,              /**< Keyboard text input */
+    SDL_KEYMAPCHANGED,          /**< Keymap changed due to a system event such as an
+                                     input language or keyboard layout change.
+                                */
 
     /* Mouse events */
     SDL_MOUSEMOTION    = 0x400, /**< Mouse moved */