For this, the core functionality of the split_dive() and
split_dive_at_time() functions were split out into new
split_dive_dont_insert() and split_dive_at_time_dont_insert(),
which do not add the new dives to the log. Thus, the undo-command
can take ownership of these dives, without having to remove them
first.
The split-dive functionality is temporarily made desktop-only
until mobile also supports "UndoObjects".
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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