chatgui: extract MainWindow constructor into sub-methods

MainWindow (~15 lines): setupUi+setupTray+setupSoundFromConfig+
setupBridgeCallbacks+setupMessaging+setupNode+setupConnects
setupSoundFromConfig(): sound config parsing + loading
setupBridgeCallbacks(): 7 gui_bridge callback registrations
setupConnects(): 5 Qt signal connections

tools/chatgui/src/mainwindow.h

@@ -42,6 +42,9 @@ private:
     void setupTray();
     void setupNode();
     void setupMessaging();
+    void setupSoundFromConfig();
+    void setupBridgeCallbacks();
+    void setupConnects();
 
     ChannelList     *m_channelList;
     MessageList     *m_messageList;