On dive editing, for every changed field the code looped through
the whole dive-list and modified the selected dives. Instead,
get the list of selected dives once and use that.
Whereas this may look like a gratuitous optimization, it will
make things easier for subsequent commits. Notably, we can
pass the list of selected dives to an "UndoObject".
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
ba9c35215e