Add virama conversion.

* src/lx-tn-im-table.h, src/lx-tn-im-table.c:
  - Add lx_tn_im_conversion() function for converting character sequences
    into precombined subjoins and conjuncts.
* ibus/engine/lx-tn-engine.c (lx_tn_engine_process_key_event):
  - Intercept key events and try to convert character sequences before
    validating input key.
  - lx_tn_engine_convert_seq() is the bridge to the actual conversion function.