Implement native reading of mo files.

Implement native reading of mo files.

Get the default language by a mix of QLocale and LyXRC::gui_language

Known limitations:
   * encoding is supposed to be UTF-8 (the charset parameter is checked);
   * context is not handled (implemented differently in LyX);
   * plural forms not implemented (not used for now in LyX);.
   * tThe byte endianness of the machine on which the .mo file have been
     built is expected to be the same as the one of the machine where this
     code is run.