Add Group concept for associating Components

It could be useful to have a special type of Component called Group, that represents a set of Components with a special identifier. This could be helpful for, among other things, diffing components that are more or less static but associated with other components within their grouping.

For example, in a voicemail app, you might have the summary card, playback controls, and some static copy. These all logically belong together, and a Group might help with that.