Make it possible to set pdomOrder in CalculusGrapherScreenView

[pixelzoom]

So that this sim can easily support alternative input in the future....

In CalculusGrapherScreenView.ts, replace this:

    this.addChild( this.graphsNode );
    this.addChild( controlPanel );
    this.addChild( toolsCheckboxGroup );
    this.addChild( resetAllButton );

... with:

    const screenViewRootNode = new Node( {
      children: [
        this.graphsNode,
        controlPanel,
        toolsCheckboxGroup,
        resetAllButton
      ]
    } );
   this.addChild( screenViewRootNode );

The reason for this is covered in detail in "Approach 2" of https://github.com/phetsims/phet-info/blob/master/doc/alternative-input-quickstart-guide.md#traversal-order. But in a nutshell, focus traversal (aka "tab order") must to be set via the pdomOrder property of a Node, and it cannot be set directly on the ScreenView. So (eventually, you don't need to do this now) the pdomOrder will be specified in CalculusGrapherScreenView.ts like this:

screenViewRootNode.pdomOrder = [
  this.graphsNode,
  controlPanel,
  toolsCheckboxGroup,
  resetAllButton
];

Let me know if you have any questions.

[pixelzoom]

I see that addChild( graphSetRadioButtonGroup ) is also called in subclasses AdvancedScreenView and LabScreenView. So in CalculusGrapherScreenView, you'll need to add:

protected readonly screenViewRootNode: Node

... and in the subclasses:

this.screenViewRootNode.addChild( graphSetRadioButtonGroup )

[pixelzoom]

I'll do this while working on #106.

[pixelzoom]

This was addressed in the above commit. @veillette ready for review.

Note that responsibility for conditionally creating graphSetRadioButtonGroup was moved to CalculusGrapherScreenView in 305f35e while working on #106.

[veillette]

Let's discussed today if you should set a flag to support alternative input in package.json or deferred to a later time.

[pixelzoom]

Yes, let's discuss. But I don't think that we should enable alternative input support yet, because of #125.

[veillette]

On 12/6, we discussed the PDOM and how we can test the PDOM in phetBrand . In #125, we will addressed the general question of how to support alternative input for the drag handler. The work done here sets the stage to make it easier to set the PDOM in CalculusGrapherScreenView, without actually implementing a PDOM order. We can close this issue.