Use key action to prevent WebPopupList firing Ajax call

[echang]

The default behavior of WebPopupList select API will trigger value change event, it could bring some problem in the case application will fire ajax call upon value change event. The workaround is to use key event API, for example:
this.Click()
Integer count = indexOfTargetElement - 1
this.GenKeyEvents("<Down : Repeat {count}>")
this.GenKeyEvents("<Enter>")