add ability to get entity by identifier

src/lib/state.h

@@ -74,6 +74,7 @@ class state {
 		state();
 		virtual ~state();
 
+		entity entity_id(uint32_t tag);
 		entity entity_at(location location);
 		vector<entity> entities_at(location location);
 		void add_entity(entity entity);
@@ -84,7 +85,7 @@ class state {
 		uint32_t resource_at(location location);
 		void set_resource(location location, uint32_t t);
 
-		int32_t get_property(uint8_t player, const string& property) const;
+		int32_t property_for(uint8_t player, const string& property) const;
 		void set_property(uint8_t player, const string& property, int32_t val);
 
 		vector<location> get_neighbours(location current);