messaging: save/restore draft messages

I have just discovered https://review.jami.net/c/ring-client-windows/+/13055.

And I wonder if the function could make sense or be understood differently.

My idea: I want to quit Jami and there is still an unsent message open, if I agree to the client suggestion, it will be saved as a draft and I can write it the next time I use Jami or finish it and then send it. The other option for users would be to discard the message when closing Jami. Of course, saving the draft message would have to be local and encrypted, as already discussed in other issues. After a restart it would be nice if Jami asks me to continue with the open message draft.

An implementation could probably be realized for all systems and clients.