WIP: Toggle secret chat

There are some TODO comments in the code. Especially how the creation of the secret chat (if there is none yet) cannot stay as it is now. But its basically already working if you're not doing "freaky" things... 😁 Lets discuss about it. I always found it ugly to have the secret chats along with the other chats in the same list. Also why should I have more than one secret chat per person. So actually I'd like to not show the secret chats on the chat list at all (at least not as separate list item). But this would need some smart notification/update/display handling for incoming secret chat messages and I guess also for new incoming secret chats.