[Editor] Create Asset object base for attachable project items

Implement GameMapAsset which contains additional metainfo related to attached GameMap.

proto/src/main/proto/project.proto

@@ -5,5 +5,14 @@ option java_outer_classname = "ProjectProto";
 
 message Project {
   required string name = 1;
-  repeated string maps = 2;
+  repeated GameMapAsset maps = 2;
+}
+
+// TODO(Add tileSetUID field)
+message GameMapAsset {
+  required string uid = 1;
+  required string source = 2;
+  required string name = 3;
+  required uint32 rows = 4;
+  required uint32 columns = 5;
 }