Play widget: expose playing and repeat

[jtpio]

Fixes #1897.

Compared to the other widgets, there is no straightforward way to control the Play widget programmatically, except for _playing attribute as discussed in #1897.

Since the animation logic is happening on the frontend, this change is mostly about triggering the click method for the corresponding buttons.

[SylvainCorlay]

I am +1 on this feature.

I am not sure actually calling the click method on the button is the way to go.

However, this looks simple enough, so am fine with merging as-is.

@jasongrout do you want to press the green button? (and closing #1897)

[jasongrout]

How about using state changes instead of messages? If we have a play state attribute that indicates whether the widget is playing or not, pause becomes setting that attribute to False, pressing play is setting that attribute to True, and clicking stop is equivalent to setting the play state to False and the value attribute to the min value.

In general, I think state changes are simpler to reason about than sending messages, and the bonus here is that we get information into what is happening on the python side.

[jasongrout]

We already have _playing, as noted in #1897. Perhaps just exposing playing and repeat as official state, and making them actually work bidirectionally would be great.

[vidartf]

The methods could also just set the state of _playing, could they not?

[jtpio]

As far as I can recall setting the state of _playing was not sufficient without significant changes to the frontend animation logic.

But yes it would be good to have both playing and repeat so it's similar to what the other widgets expose.

[jtpio]

Reworking this PR to control the Play widget by exposing and setting playing and repeat:

To trigger a stop programmatically:

play.playing = False
play.value = play.min

[jtpio]

This should also fix the issue where the play button could be hit multiple times in a row, which would "stack up" the animation.

[jtpio]

Another thing to look at in a separate PR would be to drop the pause button, and instead have a toggle play / pause button.

Edit: opened #2671

[SylvainCorlay]

This looks good to me! Thanks @jtpio.

This needs a rebase before it is merged.

[jtpio]

Done.