Doc: update reusing qml example

Make UI visible on Qt Creator.Also use id instead of parent.

Fixes: QTBUG-107030
Pick-to: 6.8 6.9
Change-Id: Ia856d214d3548e78c3b10a3e0b5e76d143986c7d
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

doc/snippets/qmlapp/qml-extending-types/components/application.qml

@@ -3,28 +3,31 @@
 //![0]
 // application.qml
 import QtQuick
-
-Column {
+Window {
+    id: root
     width: 180
     height: 180
-    padding: 1.5
-    topPadding: 10.0
-    bottomPadding: 10.0
-    spacing: 5
-
-    MessageLabel{
-        width: parent.width - 2
-        msgType: "debug"
-    }
-    MessageLabel {
-        width: parent.width - 2
-        message: "This is a warning!"
-        msgType: "warning"
-    }
-    MessageLabel {
-        width: parent.width - 2
-        message: "A critical warning!"
-        msgType: "critical"
+    visible: true
+    Column {
+        anchors.fill: parent
+        padding: 1.5
+        topPadding: 10.0
+        bottomPadding: 10.0
+        spacing: 5
+        MessageLabel{
+            width: root.width - 2
+            msgType: "debug"
+        }
+        MessageLabel {
+            width: root.width - 2
+            message: "This is a warning!"
+            msgType: "warning"
+        }
+        MessageLabel {
+            width: root.width - 2
+            message: "A critical warning!"
+            msgType: "critical"
+        }
     }
 }
 //![0]