Add "Cancel" button to cancel inference (generation)

[ThiloteE]

Add cancel button in AI summary and AI chat tabs.
Only show the cancel button, while the user is waiting for a response.

It is disputed, if the cancel button should delete the last message upon canceling and if there should be a distinction between "normal" response and "error message" response, how and where error messages could be stored and if there should be consistency between what is shown to the user in the UI and stored chat history.

See major discussion at #118 (comment)

[InAnYan]

😢 this asynchronous programming and how to properly shutdown a thread are so hard...

Need to think, this is a good feature