Merge bitcoin#15000: qt: Fix broken notificator on GNOME

c8d9d90 Fix broken notificator on GNOME (Hennadii Stepanov)

Pull request description:

  Fix bitcoin#14994; that bug was introduced in bitcoin#14228 (that was my fault).

  ~Also this commit explicit separates~ There are two functions of the tray icon:
   - a system tray widget (`QSystemTrayIcon::isSystemTrayAvailable() == true`)
   - a high-level notificator via balloon messages (`QSystemTrayIcon::supportsMessages() == true`)

  ~These properties are mutually independent,~ e.g., on Fedora 29 + GNOME:
  ```
  QSystemTrayIcon::isSystemTrayAvailable() == false;
  QSystemTrayIcon::supportsMessages() == true;
  ```

  UPDATE:

  `supportsMessages()` makes no sense without `isSystemTrayAvailable()`: `QSystemTrayIcon::showMessage()` just not working on Fedora 29 + GNOME.

Tree-SHA512: 3e75ed2dfcef112bd64b8c329227ae68ba57f3be55769629f4eb3b1c52ef1f33db635f00bb5fd57c25f73a692971d6a847ea14c525f41c594fddde6e970a8ad8

src/qt/bitcoingui.cpp

@@ -129,6 +129,7 @@ BitcoinGUI::BitcoinGUI(interfaces::Node& node, const PlatformStyle *_platformSty
     if (QSystemTrayIcon::isSystemTrayAvailable()) {
         createTrayIcon(networkStyle);
     }
+    notificator = new Notificator(QApplication::applicationName(), trayIcon, this);
 
     // Create status bar
     statusBar();
@@ -536,8 +537,7 @@ void BitcoinGUI::setClientModel(ClientModel *_clientModel)
         unitDisplayControl->setOptionsModel(_clientModel->getOptionsModel());
 
         OptionsModel* optionsModel = _clientModel->getOptionsModel();
-        if(optionsModel)
-        {
+        if (optionsModel && trayIcon) {
             // be aware of the tray icon disable state change reported by the OptionsModel object.
             connect(optionsModel, &OptionsModel::hideTrayIconChanged, this, &BitcoinGUI::setTrayIconVisible);
 
@@ -642,14 +642,10 @@ void BitcoinGUI::createTrayIcon(const NetworkStyle *networkStyle)
     assert(QSystemTrayIcon::isSystemTrayAvailable());
 
 #ifndef Q_OS_MAC
-    trayIcon = new QSystemTrayIcon(this);
+    trayIcon = new QSystemTrayIcon(networkStyle->getTrayAndWindowIcon(), this);
     QString toolTip = tr("%1 client").arg(tr(PACKAGE_NAME)) + " " + networkStyle->getTitleAddText();
     trayIcon->setToolTip(toolTip);
-    trayIcon->setIcon(networkStyle->getTrayAndWindowIcon());
-    trayIcon->hide();
 #endif
-
-    notificator = new Notificator(QApplication::applicationName(), trayIcon, this);
 }
 
 void BitcoinGUI::createTrayIconMenu()