Undo: select dives after add, remove, merge, split dive commands

Select the proper dives after the add, remove, split and merge
dives commands on undo *and* redo. Generally, select the added
dives. For undo of add, remember the pre-addition selection.
For redo of remove, select the closest dive to the first removed
dive.

The biggest part of the commit is the signal-interface between
the dive commands and the dive-list model and dive-list view.
This is done in two steps:
1) To the DiveTripModel in batches of trips. The dive trip model
   transforms the dives into indices.
2) To the DiveListView. The DiveListView has to translate the
   DiveTripModel indexes to actual indexes via its QSortFilterProxy-
   model.

For code-reuse, derive all divelist-changing commands from a new base-class,
which has a flag that describes whether the divelist changed. The helper
functions which add and remove dives are made members of the base class and
set the flag is a selected dive is added or removed.

To properly detect when the current dive was deleted it
became necessary to turn the current dive from an index
to a pointer, because indices are not stable.

Unfortunately, in some cases an index was expected and these
places now have to transform the dive into an index. These
should be converted in due course.

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

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