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/divelogimportdialog.cpp

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "desktop-widgets/divelogimportdialog.h"
 #include "desktop-widgets/mainwindow.h"
+#include "desktop-widgets/command.h"
 #include "core/color.h"
 #include "ui_divelogimportdialog.h"
 #include <QShortcut>
@@ -1010,7 +1011,7 @@ void DiveLogImportDialog::on_buttonBox_accepted()
 
 	process_imported_dives(&table, false, false);
 	autogroup_dives();
-	MainWindow::instance()->undoStack->clear();
+	Command::clear();
 	MainWindow::instance()->refreshDisplay();
 }