Initial stubs for extending the save/load version checking/upgrade code,

with support for tests on individually versioned features as well as
savegame versions.
This is mainly for improved handling of features added to patchpacks.
The current status-quo of picking a random number for the savegame
version when releasing a patch breaks loading of savegames from newer
trunk savegame versions, with or without the same patch applied, as
savegame upgrade code is not run, which can also lead to crashes on load.
Instead don't change the savegame version (but set the high bit to
keep trunk versions away), but instead include the versions of individual
features using a separate mechanism.

src/table/win32_settings.ini

@@ -17,8 +17,8 @@ static const SettingDescGlobVarList _win32_settings[] = {
 };
 #endif /* WIN32 */
 [templates]
-SDTG_BOOL = SDTG_BOOL($name,        $flags, $guiflags, $var, $def,                        $str, $strhelp, $strval, $proc, $from, $to, $cat),
-SDTG_VAR  =  SDTG_VAR($name, $type, $flags, $guiflags, $var, $def, $min, $max, $interval, $str, $strhelp, $strval, $proc, $from, $to, $cat),
+SDTG_BOOL = SDTG_BOOL($name,        $flags, $guiflags, $var, $def,                        $str, $strhelp, $strval, $proc, $from, $to, $cat, $extver),
+SDTG_VAR  =  SDTG_VAR($name, $type, $flags, $guiflags, $var, $def, $min, $max, $interval, $str, $strhelp, $strval, $proc, $from, $to, $cat, $extver),
 SDTG_END  = SDTG_END()
 
 [defaults]
@@ -33,6 +33,7 @@ load     = NULL
 from     = 0
 to       = SL_MAX_VERSION
 cat      = SC_ADVANCED
+extver   = {}