The user would be notified if the message contains "@" followed by the user's nickname or forum username. An alert would be shown, and the icon and message in the status bar would be changed. All notification is only shown if the chat window currently does not have focus.
Also added a connected_notification icon for showing in the status bar.
Each window can still watch for state changes to update the ui or to
close the window as appropriate, but for any error announcements, they
all belong in Multiplayer state now.
