Add Thai Noi sequence check based on Thai WTT 2.0

* src/Makefile.am, +src/lx-tn-im-table.h, +src/lx-tn-im-table.c:
  - Add input sequence checking table based on Thai WTT 2.0
* ibus/engine/lx-tn-engine.c:
  - Add code for retriving surrounding text and validate input key