Re-order mapgens in mainmenu and 'all settings' mapgen selection (#8705)

v6 always last to discourage selection.
Special mapgens flat, fractal, singlenode, next to last. Of these, singlenode
last to discourage selection.
Of the remaining, v5 last due to age, v7 first due to being the default.

src/mapgen/mapgen.h

@@ -102,15 +102,16 @@ private:
 	std::list<GenNotifyEvent> m_notify_events;
 };
 
+// Order must match the order of 'static MapgenDesc g_reg_mapgens[]' in mapgen.cpp
 enum MapgenType {
-	MAPGEN_V5,
-	MAPGEN_V6,
 	MAPGEN_V7,
+	MAPGEN_VALLEYS,
+	MAPGEN_CARPATHIAN,
+	MAPGEN_V5,
 	MAPGEN_FLAT,
 	MAPGEN_FRACTAL,
-	MAPGEN_VALLEYS,
 	MAPGEN_SINGLENODE,
-	MAPGEN_CARPATHIAN,
+	MAPGEN_V6,
 	MAPGEN_INVALID,
 };