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_base.cpp

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "command_base.h"
+
+namespace Command {
+
+static QUndoStack undoStack;
+
+// General commands
+void clear()
+{
+	undoStack.clear();
+}
+
+QAction *undoAction(QObject *parent)
+{
+	return undoStack.createUndoAction(parent, QCoreApplication::translate("Command", "&Undo"));
+}
+
+QAction *redoAction(QObject *parent)
+{
+	return undoStack.createRedoAction(parent, QCoreApplication::translate("Command", "&Redo"));
+}
+
+void execute(Base *cmd)
+{
+	if (cmd->workToBeDone())
+		undoStack.push(cmd);
+	else
+		delete cmd;
+}
+
+} // namespace Command
+
+