Allow access to QXcbScreen::sendStartupMessage() more directly

QX11Info::setNextStartupId() allows an application to send a startup notification when the next window is shown. Some applications, though, want to be able to send a startup notification without showing a new window (e.g. when opening a new document in an existing window).

It doesn't appear that anything in the implementation of QXcbScreen::sendStartupMessage() actually requires a new window to be created, so please consider allowing this function to be called more directly, without the need to create a new window.