Mapgen: Refactor mapgen creation and management

- Move mapgen creation logic out of EmergeManager and into Mapgen
- Internally represent mapgen type as an enum value, instead of a string
- Remove the need for a MapgenFactory per mapgen

src/mapgen_flat.h

@@ -58,6 +58,8 @@ public:
 	MapgenFlat(int mapgenid, MapgenParams *params, EmergeManager *emerge);
 	~MapgenFlat();
 
+	virtual MapgenType getType() const { return MAPGEN_FLAT; }
+
 	virtual void makeChunk(BlockMakeData *data);
 	int getSpawnLevelAtPoint(v2s16 p);
 	s16 generateTerrain();
@@ -72,16 +74,4 @@ private:
 	Noise *noise_terrain;
 };
 
-struct MapgenFactoryFlat : public MapgenFactory {
-	Mapgen *createMapgen(int mgid, MapgenParams *params, EmergeManager *emerge)
-	{
-		return new MapgenFlat(mgid, params, emerge);
-	};
-
-	MapgenSpecificParams *createMapgenParams()
-	{
-		return new MapgenFlatParams();
-	};
-};
-
 #endif