qtquick: Add example of custom tabBar

Will do some styling in a follow up

examples/qtquick/customtabbar/Guest3.qml

@@ -0,0 +1,18 @@
+/*
+  This file is part of KDDockWidgets.
+
+  SPDX-FileCopyrightText: 2020-2022 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+  Author: Sergio Martins <sergio.martins@kdab.com>
+
+  SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only
+
+  Contact KDAB at <info@kdab.com> for commercial licensing options.
+*/
+
+import QtQuick 2.9
+
+Guest {
+    anchors.fill: parent
+    background: "qrc:/assets/base.png"
+    logo: "qrc:/assets/KDAB_bubble_fulcolor.png"
+}