Where to put Conversation and MessagesModel?

[David Edmundson]

IIRC the only reason model and core were ever split was because we
were going to replace the models at some point. We did that rewrite
years ago, but then kept it as two libraries.

I think the Logger library then copied the style that was there before
of splitting everything. I think merging everything should be ok,
except it will annoy all the packagers who randomly split all the
libraries out into separate packages for no apparent reason.