gui: macOS, make appMenuBar part of the main app window

By moving the appMenuBar destruction responsibility to the QT
framework, we ensure the disconnection of the submenus signals
prior to the destruction of the main app window.

The standalone menu bar may have served a purpose in earlier
versions when it didn't contain actions that directly open
specific screens within the main application window. However,
at present, all the actions within the appMenuBar lead to the
opening of screens within the main app window. So, the absence
of a main app window makes these actions essentially pointless.

Github-Pull: gui#751
Rebased-From: bae209e387

src/qt/bitcoingui.cpp

@@ -238,7 +238,6 @@ BitcoinGUI::~BitcoinGUI()
         trayIcon->hide();
 #ifdef Q_OS_MACOS
     delete m_app_nap_inhibitor;
-    delete appMenuBar;
     MacDockIconHandler::cleanup();
 #endif
 
@@ -466,13 +465,7 @@ void BitcoinGUI::createActions()
 
 void BitcoinGUI::createMenuBar()
 {
-#ifdef Q_OS_MACOS
-    // Create a decoupled menu bar on Mac which stays even if the window is closed
-    appMenuBar = new QMenuBar();
-#else
-    // Get the main window's menu bar on other platforms
     appMenuBar = menuBar();
-#endif
 
     // Configure the menus
     QMenu *file = appMenuBar->addMenu(tr("&File"));