Nicer UI for downloading atlases

There should be a progress bar and the opportunity to cancel the download (instead of having to confirm it).
Could part of this be moved into `bg_atlasapi` ? Maybe with a `yield` statement giving updates that can be shown in the progress bar here?

Related: https://github.com/brainglobe/bg-atlasapi/issues/121

Start by locating the atlas download UI in brainrender-napari and read the related bg_atlasapi issue 121 to understand the proposed division of work. Trace how downloads currently report completion and handle confirmation, then define completion as showing progress and allowing cancellation without confirmation.

Written by the indexing model from the issue text.