Undo: implement rudimentary support for undo of dive-merging

For this, an output-parameter was added to the backend merge_dives()
function. When non-zero, instead of adding the merged dive to
the preferred trip, the preferred trip is returned to the caller.

Since the new UndoObject, just like the delete-dives UndoObject,
needs to remove/readd a set of dives, the corresponding functionality
was split-off in a helper function.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

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