Dive list: remember the current sort colum

Silly bug. We never actually remembered our current sort column when
changing the sort order.

See #518

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

qt-ui/divelistview.cpp

@@ -309,6 +309,8 @@ void DiveListView::headerClicked(int i)
 		}
 	}
 	restoreSelection();
+	// remember the new sort column
+	sortColumn = i;
 }
 
 void DiveListView::reload(DiveTripModel::Layout layout, bool forceSort)