qtquick: Add example of custom tabBar

Will do some styling in a follow up

examples/qtquick/customtabbar/MyTabBar.qml

@@ -0,0 +1,25 @@
+/*
+  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 2.6
+
+// Will be moved to a plugin in the future, if there's enough demand
+import "qrc:/kddockwidgets/qtquick/views/qml/" as KDDW
+
+Rectangle {
+    id: root
+    color: "black"
+    border.color: "orange"
+    border.width: 2
+
+    implicitHeight: 50
+
+}