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/linkgraph/linkgraphschedule.h

@@ -16,6 +16,10 @@
 
 class LinkGraphJob;
 
+namespace upstream_sl {
+	SaveLoadTable GetLinkGraphScheduleDesc();
+}
+
 /**
  * A handler doing "something" on a link graph component. It must not keep any
  * state as it is called concurrently from different threads.
@@ -42,6 +46,7 @@ private:
 	typedef std::list<LinkGraph *> GraphList;
 	typedef std::list<std::unique_ptr<LinkGraphJob>> JobList;
 	friend SaveLoadTable GetLinkGraphScheduleDesc();
+	friend upstream_sl::SaveLoadTable upstream_sl::GetLinkGraphScheduleDesc();
 
 protected:
 	std::unique_ptr<ComponentHandler> handlers[6]; ///< Handlers to be run for each job.