Instrument data labels in `graphNode`

[arouinfar]

For #190

The tree structure of graphNode currently looks like this:

Clients can hide the grid lines and tick marks, but they cannot hide the data labels that display the quantitative values.

Hiding the data labels would allow for a more qualitative experience. I don't see any need to hide the values individually, a global control like gridLines would work well here.

@pixelzoom here's a proposed tree structure. If there's a term you prefer more than dataLabels, go for it.

- graphNode
    + bars
    - dataLabels
        visibleProperty
    + gridLines 
    + tickMarks
    + visibleProperty

[pixelzoom]

Slack#DM with @arouinfar:

@pixelzoom: What I had in mind was adding graphNode.valuesVisibleProperty. Does that sound acceptable?

@arouinfar Love it

[pixelzoom]

I added graphNode.valuesVisibleProperty.

Ready for review by @arouinfar. Close if OK.

[arouinfar]

Thanks @pixelzoom, this looks great on main. I've also made a note to document this in the examples, closing.