Only load thruster & navlight nodes from the highest LOD

Re: #2110, only three months later.

Thruster and navlight nodes are only loaded form the highest detail level, and attached outside the LOD structure. It is no longer needed to duplicate these nodes for all detail levels. The amount of geometry is too small to benefit from lodding, and these nodes are visible from long ranges.
Updating the workflow now will make future changes easier (I've wanted to do some thruster refactoring, but that may take x more months).

Ideally, the old models would be cleaned and re-exported, but I made it so the extra nodes are just silently ignored in game. In modelviewer you get extra warnings.