Provide `is_playing` property

Currently there is no obvious way to determine within a callback, if the video is currently playing or not.

The existing playing property is not synchronized with the actual playback of the video as can be seen on the example page https://dash.plotly.com/dash-player Specificall, the video can be started and stopped when clicking the checkbox but when starting the video playback with the video, the property is not changed.

This is not due to the specific example as a modficiation with printing the current state of the playing variable e.g.

@app.callback(
    Output("current-time-div", "children"),
    Input("player", "currentTime"),
    State("player", "playing"),
)
def display_currentTime(currentTime, playing):
    print(playing)
    return f"Current Time: {currentTime}"

shows: The value of playing can be False while the video is in fact playing by clicking on the play button within the video.

It would be great to either have the playing property synchronized with the actual video playback or provide a new isPlaying property that reflects the actual video playing state. The same is true for the muted property (and possibly others).

plotly / dash-player

Provide `is_playing` property #72