First cut at selecting dives from the map

We'll want to enhance this: better logic for which dives are near the
selection, and it's probably best to have a "control-click" that adds
the dives to the selection rather than deselecting all the old ones.

But it's already useful.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>