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/qgstreamerplayersession.h

@@ -131,6 +131,8 @@ public slots:
     void setVolume(int volume);
     void setMuted(bool muted);
 
+    void showPrerollFrames(bool enabled);
+
 signals:
     void durationChanged(qint64 duration);
     void positionChanged(qint64 position);
@@ -209,6 +211,8 @@ private:
     qint64 m_duration;
     int m_durationQueries;
 
+    bool m_displayPrerolledFrame;
+
     enum SourceType
     {
         UnknownSrc,