alias tag for tile spec

[autofly17]

There are many dupes/obsolete tiles in the spec, so instead of removing them and breaking compatibility we could add an alias tag which would define the alias ids as copies.

;; bonus block with a dupe of 26
(tile
  (id 83)
  ...
  (aliases 26)
)

;; snow/convex tiles that are not needed anymore
;; but are aliased to 0 so levels using them don't get messed up
(tile
  (id 0)
  (aliases 2927 2932)
)

This would help with tremendously with remapping the ids back.