gstplayer: Add play/stop sync API

- Add play/stop/pause sync API
      gst_player_play_sync()
      gst_player_stop_sync()
      gst_player_pause_sync()

https://bugzilla.gnome.org/show_bug.cgi?id=778390

Upstream Status:  [i.MX specific]

Signed-off-by: Lyon Wang <lyon.wang@nxp.com>

Conflicts:
	gst-libs/gst/player/gstplayer.c
2 files changed