Add color swatches for globals to color picker

[nvkelso]

When designing a Tangram scene I set a series of color names and hex values in the global namespace for reuse later in the project.

I'd like for those colors to show up in little color swatch boxes under the main color picker UI. So if I want to change a color, I can still click on the color box inline in the source code view and it would show me my named colors (and a hover would tell me the color's variable name).

(Via support.)

[nvkelso]

Similar to: #539.