Remember the last weight used per weightsystem

With this change, if the user adds a new weightsystem to a dive, on
subsequent edits the weight amount for this weightsystem no longer
defaults to 0 but to the last weight that was used with this weightsystem.

So when the program imports a set of dives from the divecomputer and the
user starts editing them, once they enter the weight for the "integrated"
weightsystem the first time, for each of the consecutive dives that same
weight is the default once "integrated" is selected - which usually will
be the correct amount.

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