UI support for TimeTravel Debugging

[roblourens]

To close #7652, @isidorn.

DebugActionsWidget.getActions is a little awkward. Another option could be saving all the actions by name and returning a new array on each call, based on the state. That could also clean up this part:
this.actions[0] = state === debug.State.Running ? this.pauseAction : this.continueAction;

Another option could be hiding the actions dynamically.

For example,

class PauseAction {
    isVisible(): boolean {
        return something.state === State.Running;
    }
}

and the inverse for ContinueAction.

[mention-bot]

By analyzing the blame information on this pull request, we identified @egamma and @isidorn to be potential reviewers

[sandy081]

@isidorn Pull request for you :)

[isidorn]

@roblourens great work - thanks a lot 🍻

I am making the DebugActionsWidget draggable so am rewriting parts of the code there - so I will try to make the actions toggling nicer - thanks for feedback.