tabbox: Properly filter out closed windows in the stack

When TabBoxHandler::createModel() gets called, it must reset lastRaisedClient and lastRaisedClientSucc if they are closed windows. But the createModel() function currently doesn't do it, which can eventually lead to accessing dangling pointers in TabBoxHandlerPrivate::updateHighlightWindows().

Besides that, there are a couple more cases where closed windows need to be filtered out when processing the stack.