QMediaPlayer: GStreamer backend changes for playlist support.

Register type find function to check if unrecognized media is a playlist.
Emit MediaIsPlaylist error and let frontend handle playlists.

Change-Id: Ib17da1c46014ac3c0f2f48b75ea9de2f0428b573
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/plugins/gstreamer/mediaplayer/qgstreamerplayersession.h

@@ -188,6 +188,8 @@ private:
     void flushVideoProbes();
     void resumeVideoProbes();
 
+    static void playlistTypeFindFunction(GstTypeFind *find, gpointer userData);
+
     QNetworkRequest m_request;
     QMediaPlayer::State m_state;
     QMediaPlayer::State m_pendingState;
@@ -252,6 +254,8 @@ private:
     SourceType m_sourceType;
     bool m_everPlayed;
     bool m_isLiveSource;
+
+    bool m_isPlaylist;
 };
 
 QT_END_NAMESPACE