Expose `ended` property

[annahansen229]

react-player provides onEnded that is useful for doing things when the video is done playing. DashPlayer should expose an ended property that is True when the video has finished playing. I can do a workaround by comparing the currentTime to the duration but in my opinion it's not the best workaround because the callback fires every n milliseconds when using the currentTime as an Input, whereas it could just fire once if ended was exposed.