Gst player backend: don't show the first frame when resuming playback.

It's necessary to temporarily disable show-preroll-frame of video sink,
load pipeline to paused state, seek to requested position,
and after seeking is finished (position updated) playback is started
with show-preroll-frame restored.

Task-number: MOBILITY-3030
Reviewed-by: Jonas Rabbe
Change-Id: Ide1e6d909dd53f670229c293bc6be496a54e8626
(cherry picked from commit 625cce87e28fc7b5ec8785824affb3129fd3607b)
Reviewed-on: http://codereview.qt-project.org/5500
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>

src/plugins/gstreamer/mediaplayer/qgstreamerplayercontrol.h

@@ -118,6 +118,7 @@ private Q_SLOTS:
     void processEOS();
     void setBufferProgress(int progress);
     void applyPendingSeek(bool isSeekable);
+    void updatePosition(qint64 pos);
 
     void handleInvalidMedia();