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/tab-widgets/maintab.cpp

@@ -26,7 +26,7 @@
 #include "core/subsurface-string.h"
 #include "core/gettextfromc.h"
 #include "desktop-widgets/locationinformation.h"
-#include "desktop-widgets/undocommands.h"
+#include "desktop-widgets/command.h"
 
 #include "TabDiveExtraInfo.h"
 #include "TabDiveInformation.h"
@@ -799,8 +799,7 @@ void MainTab::acceptChanges()
 		updateDiveSite(ui.location->currDiveSiteUuid(), &displayed_dive);
 		copyTagsToDisplayedDive();
 
-		UndoAddDive *undoCommand = new UndoAddDive(&displayed_dive);
-		MainWindow::instance()->undoStack->push(undoCommand);
+		Command::addDive(&displayed_dive);
 
 		editMode = NONE;
 		MainWindow::instance()->exitEditState();
@@ -969,7 +968,7 @@ void MainTab::acceptChanges()
 		if (displayed_dive.when != cd->when) {
 			timestamp_t offset = cd->when - displayed_dive.when;
 			if (offset)
-				MainWindow::instance()->undoStack->push(new UndoShiftTime(selectedDives, (int)offset));
+				Command::shiftTime(selectedDives, (int)offset);
 		}
 	}
 	if (editMode != TRIP && current_dive->divetrip) {