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/animated_tile_sl.cpp

@@ -19,7 +19,7 @@
 extern std::vector<TileIndex> _animated_tiles;
 
 static const SaveLoad _animated_tile_desc[] = {
-	 SLEG_VECTOR(_animated_tiles, SLE_UINT32),
+	 SLEG_VECTOR("tiles", _animated_tiles, SLE_UINT32),
 };
 
 /**