Modified code so that unlocks store properly

Need to unlock the first level of each difficulty by default.

Also need to fix save and restore for level stats.
Fixed level unlock glitch

unlocked levels would not appear unlocked if they were after the most
recently beaten level
Game stats now are saved and loaded correctly

Delete your bin folder under MazeAndBlue (not the content folder) to use
this update.

The problem stemmed from using Dictionaries incorrectly. The objects
they return are immutable, even by their own methods, and my code had
overlooked that.