qtquick: Allow to add the guest widget declaratively

This is an alternative to setting source

examples/qtquick/dockwidgets/main.qml

@@ -33,6 +33,15 @@ KDDW.MainWindowLayout {
         source: ":/Another.qml"
     }
 
+    KDDW.DockWidget {
+        id: dock5
+        uniqueName: "dock5"
+        Rectangle {
+            id: guest
+            color: "pink"
+        }
+    }
+
     Button {
         text: "Toggle Another"
         anchors {
@@ -43,10 +52,10 @@ KDDW.MainWindowLayout {
         z: 10
 
         onClicked: {
-            if (dock4.dockWidget.visible) {
-                dock4.dockWidget.close();
+            if (dock5.dockWidget.visible) {
+                dock5.dockWidget.close();
             } else {
-                dock4.dockWidget.show();
+                dock5.dockWidget.show();
             }
         }
     }

src/private/quick/DockWidgetInstantiator.cpp

@@ -63,14 +63,21 @@ void DockWidgetInstantiator::componentComplete()
         qWarning() << Q_FUNC_INFO << "Unexpected bug.";
         return;
     }
-
-    if (m_sourceFilename.isEmpty()) {
-        qWarning() << Q_FUNC_INFO << "the 'source' property must be set.";
+    const auto childItems = this->childItems();
+    if (m_sourceFilename.isEmpty() && childItems.size() != 1) {
+        qWarning() << Q_FUNC_INFO << "Either 'source' property must be set or add exactly one child"
+                   << "; source=" << m_sourceFilename << "; num children=" << childItems.size();
         return;
     }
 
     m_dockWidget = new DockWidgetQuick(m_uniqueName);
-    m_dockWidget->setWidget(m_sourceFilename);
+
+    if (m_sourceFilename.isEmpty()) {
+        m_dockWidget->setWidget(childItems.constFirst());
+    } else {
+        m_dockWidget->setWidget(m_sourceFilename);
+    }
+
 
     Q_EMIT dockWidgetChanged();
 }

src/private/quick/DockWidgetInstantiator_p.h

@@ -34,7 +34,6 @@ class DockWidgetQuick;
 class DockWidgetInstantiator : public QQuickItem
 {
     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)