This is an interface that will allow us to treat other things as messages. We

did something similar in Talkatu to make the input widget a message and this

lets us do the same thing.
This can also be used by the HistoryAdapters when pulling history out of their

backends to make updates and other stuff easier.
This will also be used as a way to transition PurpleMessage away from using

bare strings for authors and other weird things about that API, like flags, and

stuff.
Attachments were skipped for now, as this review request is already quite large.

Ran the unit tests under valgrind.