Add DOUBLE DOT BELOW input

* src/lx-im-table.c (enum _LxCharClass):
  - Add XC_S3 (Subjoins 3) class for DOUBLE DOT BELOW.
* src/lx-im-table.c (lx_char_class):
  - Check for 0x0324 (DOUBLE DOT BELOW) and return XC_S3.
* src/lx-im-table.c (normal_action):
  - Add row and column for XC_S3.
* src/lx-kbd.c (tham_qwerty_map_lv3):
  - Change value of 0x1a20 to 0x0324 for level 3.