qt: The sound icon's menu is now a submenu of the Tools menu

Replaces (and includes) the standalone Sound gain option

src/qt/qt_mainwindow.hpp

@@ -116,6 +116,7 @@ private slots:
     void on_actionHide_tool_bar_triggered();
     void on_actionUpdate_status_bar_icons_triggered();
     void on_actionTake_screenshot_triggered();
+    void on_actionMute_Unmute_triggered();
     void on_actionSound_gain_triggered();
     void on_actionPreferences_triggered();
     void on_actionEnable_Discord_integration_triggered(bool checked);