Create a text entry for default cylinder in preferences

We don't do anything with this just yet.
And frankly, a drop down of the cylinders that we know of would be even
better. But hey, it's a start.

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

qt-ui/mainwindow.cpp

@@ -652,6 +652,7 @@ void MainWindow::readSettings()
 
 	s.beginGroup("GeneralSettings");
 	GET_TXT("default_filename", default_filename);
+	GET_TXT("default_cylinder", default_cylinder);
 	s.endGroup();
 
 	s.beginGroup("Display");