Merge #219: qt: Prevent the main window popup menu

ca5bd1c qt: Prevent the main window popup menu (Hennadii Stepanov)

Pull request description:

  bitcoin/bitcoin#11168 is not fixed by bitcoin/bitcoin#11169 completely, as users are allowed to right click on the menu bar:

  ![Screenshot from 2021-02-23 14-18-24](https://user-images.githubusercontent.com/32963518/108842753-699eb700-75e2-11eb-92ec-3aff9aa80bd4.png)

  This PR moves the context menu prohibition from `QToolBar` instance to its parent `BitcoinGUI` instance, which is derived from `QMainWindow`.

ACKs for top commit:
  jarolrod:
    ACK ca5bd1c, tested on Ubuntu 20.04 Qt 5.12. Confirming that I can replicate the behavior described on `master` and this `PR` fixes it.
  leonardojobim:
    tACK ca5bd1c

Tree-SHA512: a654ecf7ee35bb271df039be77077c1e1f9514e332587ba8622cea18da6a5b3ae8a7eb421e404ec5993c31a2f4d028e0e456fcc01facdbf61a2bc3b1e8423982

Author: MarcoFalke

src/qt/bitcoingui.cpp

@@ -88,6 +88,8 @@ BitcoinGUI::BitcoinGUI(interfaces::Node& node, const PlatformStyle *_platformSty
         move(QGuiApplication::primaryScreen()->availableGeometry().center() - frameGeometry().center());
     }
 
+    setContextMenuPolicy(Qt::PreventContextMenu);
+
 #ifdef ENABLE_WALLET
     enableWallet = WalletModel::isWalletEnabled();
 #endif // ENABLE_WALLET
@@ -544,7 +546,6 @@ void BitcoinGUI::createToolBars()
     {
         QToolBar *toolbar = addToolBar(tr("Tabs toolbar"));
         appToolBar = toolbar;
-        toolbar->setContextMenuPolicy(Qt::PreventContextMenu);
         toolbar->setMovable(false);
         toolbar->setToolButtonStyle(Qt::ToolButtonTextBesideIcon);
         toolbar->addAction(overviewAction);