Multichatpanel followup

[brichet]

Follow up #262

In the current implementation the muti-chat panel receive an openChat callback, that (1) creates the model from the chat name, and (2) open the chat in the side panel.
To avoid this back and forth (calling a method from the chat panel that perform action on the chat panel), we should probably provide a createModel method, and open the chat from the multi-chat panel itself.

Originally posted by @brichet in #262 (comment)

code change

• replace the callbacks openChat and createChat in the multichat panel by createModel
  These 2 callbacks were used to open a chat in the panel, the first one with a provided name (existing chat) and the second one without name (new chat). Both functions were supposed to call the addChat(model) function from the panel themselves, which is a bad design since they are executed from the panel.
  Instead, this PR adds a createModel callback that is responsible for creating a chat model, whether a name has been provided or not. In the case of jupyterlab-chat, the name is actually the path of the file.

• do not open the chat in the createChat commands anymore. Instead there is a new command openAndCreate that do both actions.

• add a signal when a new section is added to the multichat panel, to add the new chat to the tracker

[github-actions]

👈 Launch a Binder on branch brichet/jupyter-chat/multichatpanel_followup

[jtpio]

Thanks!