Add support for loading trunk savegames versions 293 - 299 (12.0)

Use modified upstream saveload implementation for these versions
Re-arrange headers to support multiple saveload implementations

src/saveload/upstream/airport_sl.cpp

@@ -0,0 +1,36 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file airport_sl.cpp Code handling saving and loading airport ids */
+
+#include "../../stdafx.h"
+
+#include "saveload.h"
+#include "newgrf_sl.h"
+
+#include "../../safeguards.h"
+
+namespace upstream_sl {
+
+struct APIDChunkHandler : NewGRFMappingChunkHandler {
+	APIDChunkHandler() : NewGRFMappingChunkHandler('APID', _airport_mngr) {}
+};
+
+struct ATIDChunkHandler : NewGRFMappingChunkHandler {
+	ATIDChunkHandler() : NewGRFMappingChunkHandler('ATID', _airporttile_mngr) {}
+};
+
+static const ATIDChunkHandler ATID;
+static const APIDChunkHandler APID;
+static const ChunkHandlerRef airport_chunk_handlers[] = {
+	ATID,
+	APID,
+};
+
+extern const ChunkHandlerTable _airport_chunk_handlers(airport_chunk_handlers);
+
+}