Returns the integer identifier corresponding to a listbox, choice or
radiobox selection (only if the event was a selection, not a deselection),
or a boolean value representing the value of a checkbox.
+
+ For a menu item, this method returns -1 if the item is not checkable or
+ a boolean value (true or false) for checkable items indicating the new
+ state of the item.
*/
int GetInt() const;