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() {

tools/chatgui/src/messagelist.h

@@ -18,6 +18,7 @@ public:
 
     void loadChannel(const QString& channelId);
     void refresh();
+    void saveChannelState();
     void addMessage(const QString& channelId, quint64 authorNodeId,
                     const QByteArray& content, qint64 timestamp);
 
@@ -38,7 +39,6 @@ private:
     qint64                m_savedTopMsgId = 0;
     bool                  m_wasAtBottom = false;
 
-    void saveChannelState();
     void restoreScrollPosition(const QString& channelId);
     void updateReadPosition();
 };