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 probably made sense on early versions when
it wasn't containing actions that open specific screens within the
main app window.
Right now, all the actions within the appMenuBar open screens in
the main app window, so the lack of main app window makes them
useless.

src/qt/bitcoingui.cpp

@@ -239,7 +239,6 @@ BitcoinGUI::~BitcoinGUI()
         trayIcon->hide();
 #ifdef Q_OS_MACOS
     delete m_app_nap_inhibitor;
-    delete appMenuBar;
     MacDockIconHandler::cleanup();
 #endif
 
@@ -470,13 +469,8 @@ 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"));