ObjDefManager, Mapgen SAPI: Huge refactoring

- General code cleanup
 - Unified object creation and loading
 - Specifying objects in APIs is now orthogonal (i.e. anything can take an ID,
   name string, or the raw table definition (and automatically registers if present

src/mapgen.h

@@ -213,7 +213,6 @@ public:
 
 	virtual const char *getObjectTitle() const = 0;
 
-	virtual ObjDef *create(int type) = 0;
 	virtual void clear();
 	virtual ObjDef *getByName(const std::string &name) const;