恢复显示关闭的窗口
2011-09-13在Interface Builder中，将主窗口的"Release When Closed"选项取消掉。然后在delegate里实现如下方法：
- (BOOL)applicationShouldHandleReopen:(NSApplication *)theApplication hasVisibleWindows:(BOOL)flag
{
if (flag) {
return NO;
} else {
[myWindow makeKeyAndOrderFront:self];
return YES;
}
}