Update mlterm to 3.4.2.

Changes noted in doc/en/ReleaseNote:

ver 3.4.2
* Support colorful emoji with the use of http://github.com/github/gemoji.
  (Copy images/emoji/unicode/*.png in https://github.com/github/gemoji/archive/master.zip
  to ~/.mlterm/emoji/ in advance.)
* Support DCS + q Pt ST which requests termcap/terminfo string.
* Support CSI?8452h and CSI?8452l.
* Support CSI?117h and CSI?117l.
* Add "Bidi Separators" entry to "Encoding" tab of mlconfig.
* Add "Unicode areas you won't convert to other charsets" entry to "Font" tab of mlconfig.
* Add "Word separators", "Don't scroll automatically in scrolling back" and
  "Set full width areas manually" entries to "Others" tab of mlconfig.
* Add "word_separators", "unicode_noconv_areas" and "unicode_full_width_areas"
  to OSC 5380/5381.
* Improve performance of drawing text with cairo.
* Bug fixes:
  Fix incorrect result of searching bold or italic fonts when type_engine == xcore.
  (enbugged at 3.3.0)
  Fix segfault in opening a new channel over an opened session on win32.

(tsutsui)