Duplication of buttons

[pixelzoom]

Noted while investigating #320 ...

AddAMateButton, PlayButton, and StartOverButton are 99% identical. Their only difference is in the string label. Factor out duplication, and possibly delete these classes.

[pixelzoom]

Done in the above commits.

In preparation for #319 (dynamic locale), this change also instruments the labels (Text nodes) on these buttons. So these PhET-iO elements are new:

naturalSelection.introScreen.view.environmentPanel.playButtonGroup.addAMateButton.textNode
naturalSelection.introScreen.view.environmentPanel.playButtonGroup.addAMateButton.textNode.textProperty
naturalSelection.introScreen.view.environmentPanel.playButtonGroup.playButton.textNode
naturalSelection.introScreen.view.environmentPanel.playButtonGroup.playButton.textNode.textProperty
naturalSelection.introScreen.view.environmentPanel.playButtonGroup.startOverButton.textNode
naturalSelection.introScreen.view.environmentPanel.playButtonGroup.startOverButton.textNode.textProperty

naturalSelection.labScreen.view.environmentPanel.playButtonGroup.addAMateButton.textNode
naturalSelection.labScreen.view.environmentPanel.playButtonGroup.addAMateButton.textNode.textProperty
naturalSelection.labScreen.view.environmentPanel.playButtonGroup.playButton.textNode
naturalSelection.labScreen.view.environmentPanel.playButtonGroup.playButton.textNode.textProperty
naturalSelection.labScreen.view.environmentPanel.playButtonGroup.startOverButton.textNode
naturalSelection.labScreen.view.environmentPanel.playButtonGroup.startOverButton.textNode.textProperty

By default these elements are phetioFeatured: false. Over to @amanda-phet to approve that, or make changes to overrides.js if she wants them to be featured.

[amanda-phet]

I don't think these need to be featured, so this looks good to me.

[pixelzoom]

👍🏻 closing.