Tidy up frame rate display in video examples

* Use a Column element to position QML and video frame rate counters.
* When using small-screen layouts, hide frame rate counters when dialogs
  or menus are displayed.
* Do not display instantaneous frame rate.  If changes in this rate
  trigger a repaint, then the rate itself is changed.  If the repaint
  occurs in less time than the QElapsedTimer resolution, the displayed
  instantaneous rate is always zero.
* Add a label, so it is clear which is the QML repaint rate, and which
  is the video frame rate.

Change-Id: Ie58ab162ab44bd7f1c4b297eed929b9baa73552c
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>

examples/video/qmlvideofx/qml/qmlvideofx/main-largescreen.qml

@@ -79,8 +79,6 @@ Rectangle {
                 item.anchors.top = content.top
                 item.anchors.left = content.left
                 item.anchors.right = content.right
-                item.height = 100
-                item.anchors.margins = 5
                 item.logging = root.perfMonitorsLogging
                 item.displayed = root.perfMonitorsVisible
                 item.init()
@@ -123,6 +121,7 @@ Rectangle {
                 margins: 5
             }
             buttonHeight: 32
+            topMargin: 10
         }
     }