display a sys message when a moderator clears chatroom
Profile: save main window position, save default profile when program
ends, load profile when program start
Profile.saveProfile() and Profile.loadProfile() should be non-static
ChatroomSetPane: auto update position when moving MainWindow, adjust
SettingWindow's position when user click setting button for the first time
Add IRCJtvCommandListener, ChatMessage implements this listener since all
events are directly related to change message text