factor out constants for option and var values

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Creating this issue after a Slack discussion with @samreid.

Some option and var values that could be factored out as constants. Cumulatively these might make a bit of a difference in memory footprint.

More that 1 instance of these is created (number of instances in parens):

• AxisOfSymmetryNode (2): font
• CoefficientSlider (6): map, inverseMap, tickValues, trackSize, thumbSize
• CoordinatesNode (14): font
• GQCheckbox (19): font
• GQGraphNode (4): otherCurves, decorations
• GQManipulator (4): layoutCoordinates, phetioComponentOptions
• PointToolNode (8): font
• QuadraticNode (11): font

Only 1 instance of these is created:

• DirectrixNode: font
• ExploreInteractiveEquationNode.EquationNode: font
• FocusAndDirectrixEquationNode: font, fractionFont
• FocusAndDirectrixInteractiveEquationNode.EquationNode: font
• FocusAndDirectrixModel: numberPropertyOptions
• GQScreenIconFactory: see defaults in createQuadraticNode
• NoRealRoots: font
• PointNode: layoutCoordinates
• QuadraticTermsAccordionBox: font
• StandardFormEquationNode: font
• StandardFormInteractiveEquationNode: font
• StandardFormModel: numberPropertyOptions
• VertexFormEquationNode: font
• VertexFormInteractiveEquationNode: font
• VertexFormModel: numberPropertyOptions

[pixelzoom]

Mentioning @samreid, since we had discussed the potential benefit of this on Slack, in relation to phetsims/tandem#71.

I tried factoring out constants for the default option values identified in #107 (comment). I did only the ones that are checked in that list, focusing on the classes that are instantiated more than once.

Here are the heap snapshots on Chrome 70.0.3538.77 + macOS 10.11.6, ?ea&brand=phet&fuzz, before and after factoring out these constants.

Time (minutes)	Before	After
0	44.2 MB	44.1 MB
2	51.5 MB	51.7 MB

Since there is no obvious memory reduction, I'm not going to commit the changes.

[pixelzoom]

Reopening. I think I might commit some of this anyway.