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.

builtin/settingtypes.txt

@@ -1329,7 +1329,7 @@ profiler_print_interval (Engine profiling data print interval) int 0
 #    Creating a world in the main menu will override this.
 #    Current mapgens in a highly unstable state:
 #    -    The optional floatlands of v7 (disabled by default).
-mg_name (Mapgen name) enum v7 v5,v6,v7,valleys,carpathian,fractal,flat,singlenode
+mg_name (Mapgen name) enum v7 v7,valleys,carpathian,v5,flat,fractal,singlenode,v6
 
 #    Water surface level of the world.
 water_level (Water level) int 1

src/mapgen/mapgen.cpp

@@ -81,15 +81,21 @@ struct MapgenDesc {
 //// Built-in mapgens
 ////
 
+// Order used here defines the order of appearence in mainmenu.
+// 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.
+// The order of 'enum MapgenType' in mapgen.h must match this order.
 static MapgenDesc g_reg_mapgens[] = {
-	{"v5",         true},
-	{"v6",         true},
 	{"v7",         true},
+	{"valleys",    true},
+	{"carpathian", true},
+	{"v5",         true},
 	{"flat",       true},
 	{"fractal",    true},
-	{"valleys",    true},
 	{"singlenode", true},
-	{"carpathian", true},
+	{"v6",         true},
 };
 
 STATIC_ASSERT(

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,
 };