Make it possible to set color of text labels on BarChartNode

[jessegreenberg]

They currently match the color of the bar only.

[jessegreenberg]

This is done. There is now an entry to the "bar" object called labelColor which can specify the color of the label. The default is now black, NOT the bar color. I went with this because in phetsims/energy-skate-park#240 @arouinfar said

I would be fine with doing this universally across all energy graphs. I suspect that the colors do not have sufficient contrast against light gray or white backgrounds to pass at the WCAG AAA level. https://webaim.org/resources/contrastchecker/

This would impact pendulum-lab and masses-and-springs. I kept the labels with their colors for now. @arouinfar can you confirm that the labels in these sims should be black?

[arouinfar]

@jessegreenberg yes, I think we should change the labels in pendulum-lab and masses-and-springs to black. In the graph, the text is right next to the bar so I don't think an extra level of color-cueing is necessary. These sims do have an energy legend that I think we'll need to make some design tweaks to, so I'll open an issue in the sim repos.

Color	Contrast Ratio Against White	Comments
KINETIC_ENERGY rgb( 30, 200, 45 )	2.24:1	Fail
GRAVITATIONAL_POTENTIAL_ENERGY rgb( 55, 130, 215 )	3.49:1	Fails for normal text. Passes at AA level for large text.
ELASTIC_POTENTIAL_ENERGY rgb( 0, 204, 255 )	1.89:1	Fail
HEAT_THERMAL_ENERGY rgb( 255, 85, 0 )	3.2:1	Fails for normal text. Passes at AA level for large text.
TOTAL_ENERGY rgb(180, 180, 0)	2.21:1	Fail

[jessegreenberg]

OK thanks @arouinfar - since you have made issues for the above two sims I think this issue can be closed.

[arouinfar]

@jessegreenberg I opened issues to fix the Energy Legend dialog which use the same colors as the bar graph labels, but I'd assumed the graph labels would be changed to black in this issue. Can you make this change, or should I open issues in the sim repos for that as well?

[jessegreenberg]

Ah sorry, I thought those issues included the change to the graphs as well. I can definitely do that here.

[jessegreenberg]

OK, labels have been made black. Anything else to do @arouinfar?

[arouinfar]

Looks good in master, thanks @jessegreenberg!