Option to remember model used in each chat session

[alexugthub]

Since some people might not want this, I suggest it as an option that enables the Chat to remember what model was used inside each session. I use multiple models sometimes, one for each chat, and I have to remember what model I used when I go back from a chat that used a different one.

I also open this issue so I can work on an implementation myself, and to know what others think about it.