Fix that Qt Quick Designer did not request Design mode creation

If you turned off Designer and ScxmlEditor plugins, you'd not get Design
mode even if you have Qt Quick Designer.

Fix-up of 299da01332

Change-Id: Ib3a269db9630af09c36968435ca0b0d998cc1709
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/qmldesignerplugin.cpp

@@ -203,6 +203,7 @@ bool QmlDesignerPlugin::delayedInitialize()
 
 void QmlDesignerPlugin::extensionsInitialized()
 {
+    Core::DesignMode::setDesignModeIsRequired();
     // delay after Core plugin's extensionsInitialized, so the DesignMode is availabe
     connect(Core::ICore::instance(), &Core::ICore::coreAboutToOpen, this, [this] {
         integrateIntoQtCreator(&d->mainWidget);