Split MAI KANG LAI and FINAL NGA as separate chars

* src/lx-kbd.c (tham_qwerty_map, tham_qwerty_map_lv3):
  - Change qwerty key '&' from 0x1a59 (FINAL NGA) to 0x1a58 (MAI KANG LAI).
  - Change qwerty key '"' from 0x1a56 (MEDIAL LA) to 0x1a59 (FINAL NGA).
  - Change qwerty key '}' from 0x1a57 (LA TANG LAI) to 0x1a56 (MEDIAL LA).
Swap MAIMUAN and MAIMALAI encodings

* src/lx-kbd.c (tham_qwerty_map, tham_qwerty_map_lv3):
  - Swap encodings of 0x1a71 and 0x1a72 which were previously confused
    due to different order from that of Thai.