QmlDesigner: Fix resizeToItem with live preview

The live preview recreates the "containedObject" and reparents it
to the window. But the containedObject was not tracking this.

Now we track the children accordingly.
Using bindings to keep the size in sync.

Change-Id: I051e9621f150befffec548ae0bbd431bd18480ce
Reviewed-by: Burak Hancerli <burak.hancerli@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/tools/qml2puppet/runnerconf/qmlruntime/content/resizeItemToWindow.qml

@@ -4,22 +4,46 @@ import QtQuick.Window 2.0
 import QtQuick 2.0
 
 Window {
+    id: window
     property Item containedObject: null
-    property bool __resizeGuard: false
+
+    readonly property Item firstChild: window.contentItem.children.length > 0 ? window.contentItem.children[0] : null
+
+    property bool writeGuard: false
+
+    onFirstChildChanged: {
+        window.writeGuard = true
+        window.containedObject = window.firstChild
+        window.writeGuard = false
+    }
+
     onContainedObjectChanged: {
+        if (window.writeGuard)
+            return
+
         if (containedObject == undefined || containedObject == null) {
             visible = false
             return
         }
-        __resizeGuard = true
+
-        width = containedObject.width
+        window.width = containedObject.width
-        height = containedObject.height
+        window.height = containedObject.height
+
         containedObject.parent = contentItem
-        visible = true
+        window.visible = true
-        __resizeGuard = false
+    }
+
+    Binding {
+        target: window.firstChild
+        when: window.firstChild
+        property: "height"
+        value: window.height
+    }
+
+    Binding {
+        target: window.firstChild
+        when: window.firstChild
+        property: "width"
+        value: window.width
     }
-    onWidthChanged: if (!__resizeGuard && containedObject)
-                        containedObject.width = width
-    onHeightChanged: if (!__resizeGuard && containedObject)
-                         containedObject.height = height
 }