Need a way to append to classList from ParallelDOM.js

[jessegreenberg]

For phetsims/a11y-research#158, we need an ability to modify styles of individual PDOM elements. ParallelDOM.js doesn't support this right now. The reason is that setting the style attribute on a DOM element with setAttribute will blow away any previously set styles. That is a problem because some styles are set in the scenery implementation.

Over slack, @zepumph and I considered solutions like

• Appending styles to the back of the element's css text
• setting styles to elements as property with a new ParallelDOM.js function like setPDOMStyleAttribute
• Defining a css class (using SceneryStyle) and adding a way to set the class on an element with a new ParallelDOM.js function like setPDOMClass.

Setting styles directly seems fragile, and prone to having styles overridden by scenery-internals or other things. Setting the classes on the element feels more declarative. So we like that one best. I will take a shot at implementing this.

[jessegreenberg]

Initial commit made above, would like to add some tests for this.

[jessegreenberg]

Tests added in 2510201. In 38959d2 I changed the usages of className in scenery to classList so that it is added instead of totally overridden. These usages probably wouldn't have caused problems but this is more consistent/safer.

I did some testing after this change and the PDOM is still hidden correctly and I see the classes of PDOM elements in the inspector.

@zepumph would you mind reviewing this adition?

[zepumph]

Looks really nice to me. the options paradigm is to support behavior functions, and so right now we don't support that with classLists. That seems fine to me, so I removed the option.

Nothing else here provided that you confirmed that this is successfully adding the class you need it to over in phetsims/a11y-research#158

[jessegreenberg]

Thanks for reviewing! I confirmed that it is working for the first case we needed it in GFL:B for phetsims/a11y-research#158 and phetsims/gravity-force-lab-basics#289

Closing.