Use TimeControlNode for Play/Pause/Step buttons

[jessegreenberg]

From phetsims/scenery-phet#561. A scenery-phet/TimeControlNode was created that combines the PlayPauseButton and StepForwardButton and includes PhET-iO and PDOM instrumentation. It should be used in this sim. Assigning to responsible dev.

[jessegreenberg]

The API for StepButton changed and isPlayingProperty was replaced with enabledProperty. When TimeControlNode is used in this sim the setting of enabledProperty can be removed because it is the default behavior for TimeControlNode.

[jonathanolson]

Removed from "ready-for-review", this is remaining work for the sim.