Replace a writable position() property with a seek() method.

Seeking is sometimes asynchronous, and more importantly nearly every
one using a slider for seeking ends up with a binding loop.

Change-Id: I45d92e19b0276c8b97c51c073754d1c9d3dc611e
Reviewed-by: Ling Hu <ling.hu@nokia.com>

examples/video/qmlvideo/qml/qmlvideo/VideoDummy.qml

@@ -71,4 +71,5 @@ Rectangle {
 
     function start() { }
     function stop() { }
+    function seek() { }
 }

examples/video/qmlvideo/qml/qmlvideo/VideoItem.qml

@@ -73,4 +73,5 @@ VideoOutput {
 
     function start() { mediaPlayer.play() }
     function stop() { mediaPlayer.stop() }
+    function seek(position) { mediaPlayer.seek(position); }
 }

examples/video/qmlvideo/qml/qmlvideo/VideoSeek.qml

@@ -65,7 +65,7 @@ Scene {
         }
         duration: content.contentItem() ? content.contentItem().duration : 0
         playPosition: content.contentItem() ? content.contentItem().position : 0
-        onSeekPositionChanged: { content.contentItem().position = seekPosition }
+        onSeekPositionChanged: { content.contentItem().seek(seekPosition); }
     }
 
     Component.onCompleted: root.content = content