gstplayer: Add gst_player play / stop in sync mode API

Submitted by Lyon

Description

Currently the gst_player_play() / gst_player_stop() functions are in async mode. When return from _play/stop() function, the gstplayer state might not changed yet.
When in application side, we may want to seek after gst_player_play(), and need get seekable flag in media_info. however, as the gst_player_play() function is async, the media_info might not been created yet.

So we implement gst_player_play_sync() and gst_player_stop_sync() function to wait for the state change finished. So that it will be easy to handle for application.

Do you think add these API is reasonable. or Should these functions should be implement on application side.

Thanks
Lyon

Version: 1.x