TextView/RawTextView/AttributedString

[nick-thompson]

Right now a TextView (<Text>) holds a set of RawTextView children, and during the render it constructs an AttributedString from the children text for layout and painting.

This is similar to React Native's text implementation, but I'm missing a piece for enabling the nesting of <Text> elements within each other, for cases such as bolding a single word of a sentence. Not sure exactly what the mapping needs to be between TextView / RawTextView / AttributedString, but there's surely a small tweak here that would clean up this design and enable nested TextViews.