功能：w_1向w_2传参(累加次数)并触发w_2控件cb_1(累加器)；w_2控件cb_1计算从1至所接收累加次数的累加值并返回w_1。 目的：说明窗口间相互触发控件及传参、传值的技术细节。(Function: w_1 passes to w_2 (accumulating times) and triggers w_2 control cb_1 (accumulator); w_2 control cb_1 calculates accumulative value from 1 to re