Allow multiple selections in the dive list

At this point we don't do anything with this - the commit just provides
the infrastructure changes so that this becomes possible. Subsurface
behaves the same if exactly one dive is selected and simply keeps the last
selected dive if zero or more than one dives are selected.

The goal is to be able to select multiple dives and then do actions on
them. For example pick a tank used for all of them. Or edit the location
or (yet to be implemented) other equipment data like weight carried.
And also to be able to merge multiple dives.

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