Track minimum datafile version

Add infrastructure and helper functions to track minimum datafile version.
To make this information useful we need to keep the XML and git data
format versions in track moving forward.

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

qt-ui/mainwindow.cpp

@@ -458,6 +458,8 @@ void MainWindow::closeCurrentFile()
 	free((void *)existing_filename);
 	existing_filename = NULL;
 
+	reset_min_datafile_version();
+
 	cleanUpEmpty();
 	mark_divelist_changed(false);