Rich text todos

Tracking issue for a few rough spots in #1255:

• layout may not be invalidated correctly if the env changes
• no functionality for editing the text; you have to recreate the whole object. I'm still not sure how this should interact with editing.
• default styles are confused; they currently live on the TextLayout, not on the RichText; this should probably change in some way, so that if you're using RichText we ignore anything set on the label itself.