TileStain constructor now leverages StainLevel property auto-constraints. Added some tests for this.
Added RedStain and some stain adding methods that can take the subclass instead of just specifying parameters
Removed unnecessary AddStain()
Fixing TileStain and Baddie to move from painting backgrounds to stains
Painting grid tiles now replaced with stains
Fixed AddStain so greater stains aren't replaced by lesser
Added stain decay