Split common IM utility functions as new module

* ibus/engine/Makefile.am, +ibus/engine/im-utils.h, +ibus/engine/im-utils.c,
  ibus/engine/lx-tham-engine.c, ibus/engine/lx-tn-engine.c:
  - Split common IM utility functions is_client_support_surrounding(),
    is_context_lost_key(), is_context_intact_key() into new module.