Add "LOD Override" option

Adds an "LOD Override" option to the entity and model parts to set their
LOD manually. When set to -1 (the default), the engine handles which LOD
should be used (ie normal behaviour).
Move Lod Override into the "appearance" category

I forgot to do this earlier (since I was retracing all my steps to make
these new PRs)
Merge pull request #66 from suchipi/patch-model_and_entity_parts_LOD_override

Add "LOD Override" option