ILevel and Level now have a start and finish point.
The LevelPanel now draws the start end finish point.
LevelExporter now exports the start and finish point of a level.
LevelImporter now reads the start and finish points and sets them to the level.
LevelImporter now throws an ArgumentException if the level file is not valid.
The level file is found not to be valid when any exception is raised during the
import method.
LevelEditorForm now handles the ArgumentException that is thrown when the user
attempts to load an invalid level file.