[Editor] Create MapObject model and enable its serialization

proto/src/main/proto/map.proto

@@ -29,6 +29,7 @@ message TileLayer {
 
 message ObjectLayer {
   repeated PassageAbility passageMap = 1;
+  repeated MapObject objects = 2;
 }
 
 enum PassageAbility {
@@ -36,6 +37,12 @@ enum PassageAbility {
   BLOCK = 1;
 }
 
+message MapObject {
+  required uint32 x = 1;
+  required uint32 y = 2;
+  required string code = 3;
+}
+
 message ColorLayer {
   required uint32 red = 1;
   required uint32 green = 2;