Implement Spritesheet editor functionality

[lbussell]

Todos for Spritesheets

• Spritesheet
  • Editing
    • Pencil
    • Bucket (super broken)
  • Add sprite
  • Remove sprite
  • Display
    • Data
    • Tile Grid
    • Pixel Grid
  • Export header
  • Export source
  • Export image (should be easy, not tested though tested, works)
  • undo
  • redo
  • local storage
  • Import
    • drag and drop ?
    • autotile
      • import, then crop to 256x256, then quantize each tile???
• Sprite
  • Editing
  • Display
• Sprite panel
  • New sprite form
    • implement form
    • style form
    • button for removing sprite
    • input for changing palette row
  • style list
  • allow list to scroll when it overflows the bottom of the screen

[JohnBeckner]

Sprites now have a getter that returns it's list of tiles, useful for exporting spritesheets. Tile reduction should be a stretch goal.