Planet background system

core/src/mindustry/io/JsonIO.java

@@ -163,6 +163,19 @@ public class JsonIO{
             }
         });
 
+        json.setSerializer(Planet.class, new Serializer<>(){
+            @Override
+            public void write(Json json, Planet object, Class knownType){
+                json.writeValue(object.name);
+            }
+
+            @Override
+            public Planet read(Json json, JsonValue jsonData, Class type){
+                Planet block = Vars.content.getByName(ContentType.planet, jsonData.asString());
+                return block == null ? Planets.serpulo : block;
+            }
+        });
+
         json.setSerializer(Weather.class, new Serializer<>(){
             @Override
             public void write(Json json, Weather object, Class knownType){