Fix a couple of problems with dive merging

- Dive list no longer has messed up trip entries
- Dive list is now correctly marked as modified after trips are merged
- When data file is closed, the list of selected trips is cleared which
  avoids a potential crash when loading a new data file

Fixes #286

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/mainwindow.cpp

@@ -145,6 +145,7 @@ void MainWindow::on_actionClose_triggered()
 	while (dive_table.nr)
 		delete_single_dive(0);
 
+	dive_list()->selectedTrips.clear();
 	/* clear the selection and the statistics */
 	selected_dive = -1;