Dive list: implement proper Qt-model semantics for DiveTripModel

Previously, each dive-list modifying function would lead to a
full model reset. Instead, implement proper Qt-model semantics
using beginInsertRows()/endInsertRows(), beginRemoveRows()/
endRemoveRows(), dataChange().

To do so, a DiveListNotifer singleton is generatated, which
broadcasts all changes to the dive-list. Signals are sent by
the commands and received by the DiveTripModel. Signals are
batched by dive-trip. This seems to be an adequate compromise
for the two kinds of list-views (tree and list). In the common
usecase mostly dives of a single trip are affected.

Thus, batching of dives is performed in two positions:
- At command-level to batch by trip
- In DiveTripModel to feed batches of contiguous elements
  to Qt's begin*/end*-functions.

This is conceptually simple, but rather complex code. To avoid
repetition of complex loops, the batching is implemented in
templated-functions, which are passed lambda-functions, which
are called for each batch.

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

desktop-widgets/command_divelist.h

@@ -47,10 +47,12 @@ private:
 	bool workToBeDone() override;
 
 	// For redo
-	DiveToAdd	diveToAdd;
+	// Note: we use a vector even though we add only a single dive, so
+	// that we can reuse the multi-dive functions of the other commands.
+	std::vector<DiveToAdd>	divesToAdd;
 
 	// For undo
-	dive		*diveToRemove;
+	std::vector<dive *>	divesToRemove;
 };
 
 class DeleteDive : public Base {