Provide feedback that the importer is still running and basic status

It would be nice to be able to see what beets is doing and to have some confirmation that it’s still active, even without the 'verbose' flag.

One way to do this would be to have some basic status output in normal mode, for example:
“importing…”
“scanning AcoustIDs…”

and possibly the use of the old spinning cursor trick, with curses /|-

Of course, this depends on how quiet beets is supposed to be in normal mode, but it would be nice, assuming it’s viable.