chatgui: save scroll/read position on app quit via aboutToQuit

Make saveChannelState() public, connect to QApplication::aboutToQuit so
state is persisted on SIGINT, tray quit, or any other shutdown.

tools/chatgui/src/mainwindow.cpp

@@ -229,6 +229,8 @@ void MainWindow::setupConnects() {
                     m_channelList->setChannelUnreadCount(channelId,
                         m_db->getUnreadCount(channelId, lastReadMsgId));
             });
+
+    connect(qApp, &QApplication::aboutToQuit, m_messageList, &MessageList::saveChannelState);
 }
 
 void MainWindow::setupTray() {