Add an explicit attribute order for tags in the World Model.

It would be nice to have a fixed attribute order for tags in the World Model.
```xml
<balls>
    <ball markerX="-111.825" y="-106.313" markerY="-106.313" rotation="0" startRotation="0" id="{0aaf8620-a0b8-48c7-9679-7b44df7cd753}" x="-111.825"/>
</balls>
```
For example, we could set them up in the following order: `x`, `y`, `markerX`, `markerY`, `rotation`, `startRotation`, `id`.