Play manual addition of dives via an UndoCommand. Since this does in
large parts the same thing as undo/redo of dive deletion (just the
other way round and only a single instead of multiple dive), factor
out the functions that add/delete dives and take care of trips.
The UI-interaction is just mindless copy&paste and will have to
be adapted.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
