One-on-one chat fixes

Replaces #268. See comments there.
random obj vs. `Candy.Core.ChatRoom`

@ test coverage: yeah that's a good idea, now that we have finally the possibility :)

@ breaking changes: I have to see what we already did so far, but I think it might be worth doing a Candy 2.0. Therefore breaking changes are doable. But I'm not 100% sure yet.
Regarding plugins: depending on the amount of changes necessary we probably have to invest some time and fix them before releasing. Still manageable currently IMHO.