Don't create tray icon if system tray is not supported

This fixes the problem that the tray icon is created in GNOME if it was
enabled in other desktop environment previously.

src/controller.cpp

@@ -294,7 +294,7 @@ bool Controller::supportSystemTray() const
 void Controller::setQuitOnLastWindowClosed()
 {
 #ifndef Q_OS_ANDROID
-    if (NeoChatConfig::self()->systemTray()) {
+    if (supportSystemTray() && NeoChatConfig::self()->systemTray()) {
         m_trayIcon = new TrayIcon(this);
         m_trayIcon->show();
     } else {