Energy Legend: make all text black and add color swatches

[arouinfar]

In phetsims/griddle#49, labelColor was added to bar graphs, and the is default black. The primary motivation for this change was the contrast ratio between the colored text and the white background was poor (see values in phetsims/griddle#49 (comment)).

Another place that the colored text appears is in the Energy Legend:

All text should be made black, but to compensate, we should add a color swatch next to each entry.