GameObects need to have a parent

[pathunstrom]

In an effort to support relative positioning inside of #577 we're going to need to break a fundamental assumption of ppb: objects need to know the container they live in. Since we have the gomlib now, we don't need to worry about this.

Add a self._parent as a weak reference to the containing GameObject, then when a GameObject is added to another, set the child._parent to the container. When you remove a GameObject from another, set that reference to None.

[AstraLuma]

I believe @avarice-m said in discord faer would be working on this.

[jonesmat]

@pathunstrom does this still need to be done?