Undo: isolate undo-commands

This refactors the undo-commands (which are now only "commands").

- Move everything in namespace Command. This allows shortening of
  names without polluting the global namespace. Moreover, the prefix
  Command:: will immediately signal that the undo-machinery is
  invoked. This is more terse than UndoCommands::instance()->...
- Remove the Undo in front of the class-names. Creating an "UndoX"
  object to do "X" is paradoxical.
- Create a base class for all commands that defines the Qt-translation
  functions. Thus all translations end up in the "Command" context.
- Add a workToBeDone() function, which signals whether this should be
  added to the UndoStack. Thus the caller doesn't have to check itself
  whether this any work will be done. Note: Qt5.9 introduces "setObsolete"
  which does the same.
- Split into public and internal header files. In the public header
  file only export the function calls, thus hiding all implementation
  details from the caller.
- Split in different translation units: One for the stubs, one for
  the base classes and one for groups of commands. Currently, there
  is only one class of commands: divelist-commands.
- Move the undoStack from the MainWindow class into commands_base.cpp.
  If we want to implement MDI, this can easily be moved into an
  appropriate Document class.

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

desktop-widgets/command.cpp

@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "command.h"
+#include "command_divelist.h"
+
+namespace Command {
+
+// Dive-list related commands
+void addDive(dive *d)
+{
+	execute(new AddDive(d));
+}
+
+void deleteDive(const QVector<struct dive*> &divesToDelete)
+{
+	execute(new DeleteDive(divesToDelete));
+}
+
+void shiftTime(const QVector<dive *> &changedDives, int amount)
+{
+	execute(new ShiftTime(changedDives, amount));
+}
+
+void renumberDives(const QVector<QPair<int, int>> &divesToRenumber)
+{
+	execute(new RenumberDives(divesToRenumber));
+}
+
+void removeDivesFromTrip(const QVector<dive *> &divesToRemove)
+{
+	execute(new RemoveDivesFromTrip(divesToRemove));
+}
+
+void removeAutogenTrips()
+{
+	execute(new RemoveAutogenTrips);
+}
+
+void addDivesToTrip(const QVector<dive *> &divesToAddIn, dive_trip *trip)
+{
+	execute(new AddDivesToTrip(divesToAddIn, trip));
+}
+
+void createTrip(const QVector<dive *> &divesToAddIn)
+{
+	execute(new CreateTrip(divesToAddIn));
+}
+
+void autogroupDives()
+{
+	execute(new AutogroupDives);
+}
+
+void mergeTrips(dive_trip *trip1, dive_trip *trip2)
+{
+	execute(new MergeTrips(trip1, trip2));
+}
+
+void splitDives(dive *d, duration_t time)
+{
+	execute(new SplitDives(d, time));
+}
+
+void mergeDives(const QVector <dive *> &dives)
+{
+	execute(new MergeDives(dives));
+}
+
+} // namespace Command