Initial support for loading savegames from SpringPP v2.0.102/103

Savegame version 220

Bin all extra settings added in SpringPP.
Bin extra features: trip histories, more conditional orders/jump counter,
prev order time, snow-line chunk, town auto adv. campaign, etc.
Fix vehicle flags.
Fix bridges wrt height changes.
Handle 60-bit RIFF chunk sizes.
Doesn't deal with huge airports, loading games with these may
lead to undefined ehaviour.
PAX signals and traffic lights are ignored.

src/saveload/extended_ver_sl.h

@@ -36,6 +36,11 @@ enum SlXvFeatureIndex {
 	XSLFI_INFRA_SHARING,                          ///< Infrastructure sharing patch
 	XSLFI_VARIABLE_DAY_LENGTH,                    ///< Variable day length patch
 
+	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
+	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk
+	XSLFI_MIGHT_USE_PAX_SIGNALS,                  ///< This save game might use the pax-signals feature
+	XSLFI_SPRINGPP,                               ///< This is a SpringPP game, use this for loading some settings
+
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };