qtquick: Add an example of how to instantiate a dock widget from QML

Instead of using C++

src/private/quick/DockWidgetInstantiator_p.h

@@ -0,0 +1,64 @@
+/*
+  This file is part of KDDockWidgets.
+
+  SPDX-FileCopyrightText: 2019-2020 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+  Author: Sérgio 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.
+*/
+
+#include "DockWidgetQuick.h"
+
+#include <QObject>
+#include <QQmlParserStatus>
+#include <QString>
+
+namespace KDDockWidgets {
+
+class DockWidgetQuick;
+
+/**
+ * @brief Indirection helper to instantiate dock widgets from QML
+ *
+ * "DockWidget {}" in QML won't create a KDDockWidget::DockWidget directly, but instead an
+ * DockWidgetInstantiator. DockWidgetInstantiator will then create the DockWidget instance only
+ * when the QML parsing ends (and all propreties are set)
+ *
+ * This allows to pass the correct uniqueName to DockWidget's ctor.
+ */
+class DockWidgetInstantiator
+        : public QObject
+        , public QQmlParserStatus
+{
+    Q_OBJECT
+    Q_INTERFACES(QQmlParserStatus)
+    Q_PROPERTY(QString uniqueName READ uniqueName WRITE setUniqueName NOTIFY uniqueNameChanged)
+    Q_PROPERTY(QString source READ source WRITE setSource NOTIFY sourceChanged)
+    Q_PROPERTY(KDDockWidgets::DockWidgetQuick *dockWidget READ dockWidget NOTIFY dockWidgetChanged)
+public:
+    QString uniqueName() const;
+    void setUniqueName(const QString &);
+
+    QString source() const;
+    void setSource(const QString &);
+
+    DockWidgetQuick *dockWidget() const;
+
+protected:
+    void classBegin() override;
+    void componentComplete() override;
+
+Q_SIGNALS:
+    void uniqueNameChanged();
+    void sourceChanged();
+    void dockWidgetChanged();
+private:
+    QString m_uniqueName;
+    QString m_sourceFilename;
+    DockWidgetQuick *m_dockWidget = nullptr;
+
+};
+
+}