Add API changelogs

[GUI]

I think we should start maintaining public CHANGELOGs for our individual APIs. In a variety of cases, we continue to add new features to a specific version of an API (while maintaining backwards compatibility), but we don't necessarily do a good job announcing those enhancements or making the history of the API publicly available. While we could do a lot more to announce changes, a simple CHANGELOG of dates and when things happened for each API would be a good, simple place to start.