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/mainwindow.h

@@ -34,7 +34,6 @@ class DivePlannerWidget;
 class ProfileWidget2;
 class PlannerDetails;
 class PlannerSettingsWidget;
-class QUndoStack;
 class LocationInformationWidget;
 
 typedef std::pair<QByteArray, QVariant> WidgetProperty;
@@ -82,7 +81,6 @@ public:
 	void setApplicationState(const QByteArray& state);
 	void setStateProperties(const QByteArray& state, const PropertyList& tl, const PropertyList& tr, const PropertyList& bl,const PropertyList& br);
 	bool inPlanner();
-	QUndoStack *undoStack;
 	NotificationWidget *getNotificationWidget();
 	void enableDisableCloudActions();
 	void setCheckedActionFilterTags(bool checked);