PhET-iO Design Review for 1.1 release

[arouinfar]

Related to #108

We would like to republish Density with the latest PhET-iO features and functionality.

The PhET-iO design took place a while ago, so It could use a refresh.

• [x] Review dynamic strings/locale, #145
• [x] Review tree structure
• [x] Update phetioFeatured behavior, if necessary

• 151

[arouinfar]

I completed the tree review with @Nancy-Salpepi and all resulting issues have been reported above. No updates to the phetioFeatured overrides are necessary.

[arouinfar]

The only remaining item is the examples doc, which is tracked in its own issue. Closing.