QmlDesigner: Work around layout issue

Layout.fillWidth: true does not work without an explicit size.

Change-Id: I4308016febddef1735d6e5261ac5bc989496bac9
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

share/qtcreator/qmldesigner/propertyEditorQmlSources/QtQuick/ItemPane.qml

@@ -117,6 +117,7 @@ Rectangle {
                             placeholderText: qsTr("id")
                             text: backendValues.id.value
                             Layout.fillWidth: true
+                            width: 240
                             showTranslateCheckBox: false
                             showExtendedFunctionButton: false
                             enabled: !modelNodeBackend.multiSelection