Undo: clear undo history on load / import

Keeping undo-history across load makes little sense. The user was
expressly reminded that they have unsaved work.

For import (from other logs or the dive-computer) an undo-functionality
would be desirable. Nevertheless, this is rather complex since
new and old dives are merged. Implementation would require a finer
backend<->undocommand interface. Thus, leave this for now until more
experience with the undo system is acquired.

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

desktop-widgets/divelogimportdialog.cpp

@@ -7,6 +7,7 @@
 #include <QDrag>
 #include <QMimeData>
 #include <QRegExp>
+#include <QUndoStack>
 #include "core/qthelper.h"
 #include "core/import-csv.h"
 
@@ -1008,6 +1009,7 @@ void DiveLogImportDialog::on_buttonBox_accepted()
 	}
 
 	process_imported_dives(&table, false, false);
+	MainWindow::instance()->undoStack->clear();
 	MainWindow::instance()->refreshDisplay();
 }