MakeTiles: allow non-integer tile names; generate an idx file
tiles: keep a mapping of tile names to the tile spec
refactor: make TileSpec do its own reference resolving
tiles: fix syntax of named tiles
tiles: add name property to spec
tiles: parse 'building' attribute at initialize time

rather than per-use
file-format: use tile names instead of numbers (write only)
file-format: use tile names instead of numbers (load)

in addition, use :pwr suffix for any tile where the PWR bit is set
file-format: throw an exception if an unrecognized tile id is seen