Added levers beeing placeable on halfslabs

Don't merge this yet. I don't know if this is okay.
Are you sure this condition is right? I'd say you need to check if `a_RelY < cChunkDef::Height` instead, you're placing the lever on top of the slab, after all.
Also, missing parenthesis around the condition, and if you break the condition into separate lines, then break each and every segment..