Documentation. More misc. changes.

* Game.cs:
  Documentation.
  The default scene is now set according to GameSettings.DefaultScene.
  Added checking to generic GetScene() method.

* GameSettings.cs:
  Added DefaultScene field.

* Program.cs:
  Set default scene to "menu".

* Extensions.cs, Parameters.cs:
  Documentation.

* Button.cs:
  Documentation.
  Added handling of the button not being enabled when Reset() is called.

* ButtonList.cs:
  Documentation.
  Better handling of MaxIndex.