Calm down the Globe widget

This addresses two issues:
- the kMessageWidget did not word wrap, so if it was wider then the Globe
  widget the Globe would expand (and divelist shrink) when the user
  switched from a dive with location to a dive without location
- the code was also too aggressive removing and redrawing the message
  widget when switching from dive to dive. with this change we only hide
  the widget if the next dive has a location (and only show it if it isn't
  shown already). We also hide the message if no dive is selected.

Reported-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>