It's unfortunate to need to have access to a GPUI window in order to
create a terminal, because it forces to take a `Window` parameter in
entities that otherwise would have been pure models.

This pull request changes it so that we pass the `Project`'s entity id,
which is equally stable as the window id.

Release Notes:

- N/A

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>