gstplayer: Add gst_player_get_state() API

- Add gst_player_get_state() API

https://bugzilla.gnome.org/show_bug.cgi?id=778379
Upstream-Status: Submitted

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

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