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/depot_sl.cpp

@@ -20,7 +20,7 @@ static TownID _town_index;
 static const SaveLoad _depot_desc[] = {
 	 SLE_CONDVAR(Depot, xy,         SLE_FILE_U16 | SLE_VAR_U32, SL_MIN_VERSION, SLV_6),
 	 SLE_CONDVAR(Depot, xy,         SLE_UINT32,                 SLV_6, SL_MAX_VERSION),
-	SLEG_CONDVAR(_town_index,       SLE_UINT16,                 SL_MIN_VERSION, SLV_141),
+	SLEG_CONDVAR("town_index", _town_index, SLE_UINT16,       SL_MIN_VERSION, SLV_141),
 	 SLE_CONDREF(Depot, town,       REF_TOWN,                 SLV_141, SL_MAX_VERSION),
 	 SLE_CONDVAR(Depot, town_cn,    SLE_UINT16,               SLV_141, SL_MAX_VERSION),
 	SLE_CONDSSTR(Depot, name,       SLE_STR,                  SLV_141, SL_MAX_VERSION),