Unused variables in function calls can be commented out. Unused
return variables get a new routine `ignore_unused()` that silences the
warnings with zero overhead.
This moves EESchema DLIST structures to rtree. These changes are more
fundamental than the pcbnew changes from 9163ac543888c01d11d1877d7c1
and 961b22d60 as eeschema operations were more dependent on passing
drawing list references around with SCH_ITEM* objects.