Improve the `tilesAround:` method + create a "Map" class to take care of the details?

`tilesAround:` is broken if you are near the edges of the map (lower bound is OK, upper bound needs to be done).

Maybe we should have a map object so we can have a solid way of telling the `RTGridLayout` the number of columns it needs to display. This is easy while the center tile is not near the edge of the map.
