docs: def byLexer( lexer ):
          """
          Looks up the language plugin by the lexer class of the inputed lexer.
          
          :param      lexer | <QsciLexer>
          
          :return     <XLanguage> || None
          """
          XLanguage.load()
          
          lexerType = type(lexer)
          for lang in XLanguage._plugins.values():
              if ( lang.lexerType() == lexerType ):
                  return lang
          return None