Add support for scaled subjoined (1A5B..1A5E)

* src/lx-tham-kbd.c (tham_qwerty_map_lv3, tham_keycode_map[3]):
  - Change level 3 for LOW PA, MA, BA, HIGH SA to corresponding scaled
    subjoins (U+1A5A, U+1A5C, U+1A5D, U+1A5E respectively).
* src/lx-tham-im-table.c (enum _LxThamCharClass, charclass, normal_action):
  - Add S4 char class for scaled subjoins
  - Add actions to action table.
* src/lx-tham-im-table.c (lx_tham_im_preedit_action):
  - Also accept & swap S4 char class after LV.