Feature: framework to make savegames self-descriptive

We won't be able to make it fully self-descriptive (looking at you
MAP-chunks), but anything else can. With this framework, we can
add headers for each chunk explaining how each chunk looks like
in detail.

They also will all be tables, making it a lot easier to read in
external tooling, and opening the way to consider a database
(like SQLite) to use as savegame format.

Lastly, with the headers in the savegame, you can freely add
fields without needing a savegame version bump; older versions
of OpenTTD will simply ignore the new field. This also means
we can remove all the SLE_CONDNULL, as they are irrelevant.

The next few commits will start using this framework.

src/saveload/linkgraph_sl.cpp

@@ -83,7 +83,7 @@ public:
 		    SLE_VAR(Node, demand,      SLE_UINT32),
 		    SLE_VAR(Node, station,     SLE_UINT16),
 		    SLE_VAR(Node, last_update, SLE_INT32),
-		SLEG_STRUCTLIST(SlLinkgraphEdge),
+		SLEG_STRUCTLIST("edges", SlLinkgraphEdge),
 	};
 
 	void Save(LinkGraph *lg) const override
@@ -118,9 +118,9 @@ SaveLoadTable GetLinkGraphDesc()
 {
 	static const SaveLoad link_graph_desc[] = {
 		 SLE_VAR(LinkGraph, last_compression, SLE_INT32),
-		SLEG_CONDVAR(_num_nodes,              SLE_UINT16, SL_MIN_VERSION, SLV_SAVELOAD_LIST_LENGTH),
+		SLEG_CONDVAR("num_nodes", _num_nodes, SLE_UINT16, SL_MIN_VERSION, SLV_SAVELOAD_LIST_LENGTH),
 		 SLE_VAR(LinkGraph, cargo,            SLE_UINT8),
-		SLEG_STRUCTLIST(SlLinkgraphNode),
+		SLEG_STRUCTLIST("nodes", SlLinkgraphNode),
 	};
 	return link_graph_desc;
 }
@@ -165,7 +165,7 @@ SaveLoadTable GetLinkGraphJobDesc()
 	static const SaveLoad job_desc[] = {
 		SLE_VAR(LinkGraphJob, join_date,        SLE_INT32),
 		SLE_VAR(LinkGraphJob, link_graph.index, SLE_UINT16),
-		SLEG_STRUCT(SlLinkgraphJobProxy),
+		SLEG_STRUCT("linkgraph", SlLinkgraphJobProxy),
 	};
 
 	/* The member offset arithmetic below is only valid if the types in question