Follow up UX improvments

[brichet]

#298 brings changes on UI/UX in the chat widget, but it also removed some existing features, that may require some more discussion.

Here is a non exhaustive list that can be updated:

• Edit button, allowing you to edit a message after it has been sent. Some discussion about it here and here

  

• Send with dropdown, that was useful to add cell content or text selection in the message

  

• Message deleted indicator. Before #298, the message header was kept when a message had been deleted, to display it.

  

[brichet]

• Edit button, allowing you to edit a message after it has been sent. Some discussion about it here

This edit button is useful in the context of a regular chat (without AI involved).

As mentioned in this comment, the issue related to AI seeing edited messages as new message should be fixed with #299.

An other option could be to make the edit button configurable, and disable it from jupyter-ai.

[brichet]

• Send with dropdown, that was useful to add cell content or text selection in the message

Thanks to #248 it is now possible to drag and drop a cell as attachment.
#253 would allow to attach text selection as well.

These solutions require a pointer device to attach cell or selection (using drag and drop).
If it is not restored by default, the Send with drop-down may be optional, as it remains useful for accessibility reasons.