Don't use SystemTray integration on GNOME and ElementaryOS

These platforms don't support it so hiding NeoChat in the tray in these
platforms is not a good idea and other a rather poor user experience.

(cherry picked from commit 13888401fa)

src/controller.cpp

@@ -429,7 +429,8 @@ bool Controller::supportSystemTray() const
 #ifdef Q_OS_ANDROID
     return false;
 #else
-    return true;
+    QString de = getenv("XDG_CURRENT_DESKTOP");
+    return de != QStringLiteral("GNOME") && de != QStringLiteral("Pantheon");
 #endif
 }