gstplayer: Add gst_player_get_state API

Submitted by Lyon

For gstpalyer state, currently we can only get the state by state_change callback, when mainloop start running.

However, if we need to get the current state when mainloop has not started running. There is no way to get the gstplayer state.

So considering add this gst_player_get_state() API to get the current player state.

Version: 1.x