[Mac] Fire ComboBox.SelectionChanged when item selected in code
[Mac] Fire ComboBox.SelectionChanged when item selected in code

When the NSPopUpButton.SelectItem(nint index) is called the
ComboBox.SelectionChanged event is now fired. This allows UI automation
tools to select the combo box item programatically and still allow the
XWT SelectionChanged event to be triggered.

antmicro/xwt,mono/xwt