Looks up the language plugin by the lexer class of the inputed lexer.
        
        :param      lexer | <QsciLexer>
        
        :return     <XLanguage> || None

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