设计规则：假设有一个字符串为“ABCDE”, 每按下一次任意键(响应WM_KEYDOWN消息)，该字符串就往前移动一格，当字符串的第一个字符碰到窗口的用户区域的边界时，该字符串就反方向移动。字符串每移动一格，颜色将随机地发生变化。

　　提示：假如初始状态设定字符串在（100，100）位置上，开始时A字符作为首字符向左移动，则新的位置变成(100+Δx,100),设Δx -10，重新显示整个字符串。当首字符的坐标位置小于0时，则Δx +10，字符串向右移动。说明Px和Py为两个静态的全局变量，用以记录当前字符串的坐标位置, Plen记录字符串的长度。所用到的API函数可以参照作业一和作业二。

-Design rule: Suppose there is a string　ABCDE , each press any key (in response to WM_KEYDOWN message), the string to move forward a grid, when the first character string hit the user area of the window Boundary, the string moves in the opposite direction. As each string moves, the color will change randomly.

Note: If the initial state setting string is in the (100,100) position, and the A character starts to move to the left as the first character, the new position becomes (100+ Δx, 100), and Δx　-10. Displays the entire string. When the coordinate position of the first character is less than 0, Δx　+ 10, and the character string moves to the right. Descr iption Px and Py are two static global variables used to record the coordinates of the current position of the string, Plen record the length of the string. The API functions used can refer to job one and job two.