Undo: use QUndoStack::isClean() to determine unsaved changes

Properly implement the unsaved-changes flag(s). Since we currently have
two kinds of changes, there are two flags:
1) dive_list_changed in divelist.c marks non-undoable changes. This flag
   is only cleared on save or load.
2) QUndoStack::isClean() is used to determine the state of undoable
   changes. Every time the user returns to the state where they saved,
   this flag is cleared.

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

desktop-widgets/command_base.cpp

@@ -1,17 +1,33 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #include "command_base.h"
+#include "core/qthelper.h" // for updateWindowTitle()
 
 namespace Command {
 
 static QUndoStack undoStack;
 
 // General commands
+void init()
+{
+	QObject::connect(&undoStack, &QUndoStack::cleanChanged, &updateWindowTitle);
+}
+
 void clear()
 {
 	undoStack.clear();
 }
 
+void setClean()
+{
+	undoStack.setClean();
+}
+
+bool isClean()
+{
+	return undoStack.isClean();
+}
+
 QAction *undoAction(QObject *parent)
 {
 	return undoStack.createUndoAction(parent, QCoreApplication::translate("Command", "&Undo"));