tiles: add name property to spec
MakeTiles: refactor to simplify main() method body
MakeTiles: allow non-integer tile names; generate an idx file
MakeTiles: write unique index for each output file
MakeTiles: remove support for width/height specifier
MakeTiles: output to a graphics directory instead of a specific file
graphics: change where images are loaded from when running the gui

tiles.png used to be at the root of the jar,
now it is in the '16x16' subdirectory (or '32x32' or '8x8' or 'sm'
depending on what resolution is being loaded)