Add a tutorial task to use the minimap

From #610, Michael's idea to teach how to navigate with the minimap, and also reinforce the use of layers:

> The point of the minimap is that you can navigate relatively long distances quickly, while staying zoomed in. So, we need to observe something in detail (zoomed in), over a vast area.
> Could we do some kind of "Where's Wally?"
> And maybe Wally only likes to hang out at certain elevations.

The tutorial code (https://github.com/a-b-street/abstreet/blob/master/game/src/sandbox/gameplay/tutorial.rs) is on the more complex side, but this'd make a decent first issue to learn the UI.