Replace "chat valid" in chatlistmodel with "isopen" in chat entity

The problem here was that chat list was holding the state of the currently opened chat and needed additional lines of code to handle this (chat valid?) - But, in fact this should be solely the property of the chat itself. So hence this changes.

That actually fixes the problem of the last message getting marked as read (#235 (closed)) when the chat just has been left, and new messages arrive.