* Edit Mode now uses the Level object itself to keep the drawing data
rather than pull its Palette and Chunks out, so it can hang on to more
information. The Canvas widget is given references to the
Level.Palette and Level.Chunker via Canvas.LoadLevel()
* Fix the handoff between Edit Mode and Play Mode. They pass the Level
object back and forth and the Filename, because it's not part of the
Level. You can save the map with its original settings after returning
from Play Mode.
* Fix the collision detection in Play Mode. It broke previously when
palettes were added because of the difference between a render.Point
and a level.Pixel and it couldn't easily look up coordinates. The new
Chunker system provides a render.Point lookup API.
* All pixels are solid for collision right now, TODO is to return Swatch
information from the pixels touching the player character and react
accordingly (non-solid, fire flag, etc.)
* Remove the level.Grid type as it has been replaced by the Chunker.
* Clean up some unused variables and functions.
