FR: Allow renderers to window properties like title and footer

[chrisgrieser]

As opposed to #200, this FR is about being able to set it as a render render function.

So basically, I settle on a wrapped version of minimal, and used the pointers from #200 to set the title into the window border:

However, such a solution requires the user to add some custom config to on_open, on top of selecting a specific renderer to got with it, which I think is far from ideal. It would of course be better if the user would only have to select a renderer. On top of that, setting the title via on_open has some drawbacks, for instance if the title is longer than the message, then the title gets cut off, etc.

Especially with nvim 0.10's support for footer, this is relevant, since I think the footer feels like the perfect place for stuff like the notification time.