Allow pruning/deleting of entire branches of a chat.

As far as I am aware, one has to delete each message of a branch one by one, to delete the branch. And even then, the empty branch is shown.

In the same way, in a multi-model chat, it doesn’t currently seem to be possible to delete one of the models completely - each response must be deleted one by one, and even then, the empty model remains, rather than the chat reverting to a single-model conversation.