Dynamically order nodes on z axis

[lukors]

Selecting a node should bring it to the top of the z-stack, and all other nodes down.

There needs to be an ordering system to do this.

Maybe something like a Res<Vec<NodeId>>, and every time a node gets selected it gets removed and pushed onto the top. Then a system that sets the z-axis translation every time that resource has been updated. If the NodeId does not yet exist it just gets pushed on the top. If any NodeId is missing in the system that sets the z-axis translation, it gets removed from the Vec.