Reuse map windows

Instead of opening a new window / new map for every dive site we now have only
two windows / maps. One for the overview of all the dive sites, the other one
that is used to show the map for a specific dive site.

This also switches the position preference from CENTER to MOUSE - this way it's
less likely that the two map windows will be drawn exactly on top of each
other.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>