qtquick|examples: Don't call them "MainWindow"

Rather "main layout", as they are not top-levels, unlike for
QWidgets

As suggested in issue #196

examples/qtquick/customtitlebar/main.qml

@@ -22,6 +22,6 @@ ApplicationWindow {
         id: dockWidgetArea
         anchors.fill: parent
 
-        uniqueName: "MyWindowName-1"
+        uniqueName: "MyMainLayout"
     }
 }