Map keyboard with scan code instead of key value

* src/lx-kbd.h, src/lx-kbd.c (lx_map_keycode):
  - Add keycode-to-unichar mapping function.
* ibus/engine/engine.c (ibus_lanxang_engine_process_key_event):
  - Call lx_map_keycode() instead of lx_map_key() when translating keys.